docs: add project README

README.md

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+# nff
+
+This is a high performance configuration parser and written in Rust. The goal is
+to receive possibly jumbled up nftables rule files, and output ✨ pretty ✨
+human readable output in return. The main emphasis is on the syntax-aware
+formatting with comprehensive grammar support. It is a future goal to allow
+editors to hook into nff in order to format your rulesets directly from your
+editor, or as a diagnostics source.
+
+## Features
+
+nff is in its early stages of development. While _most_ of the syntax is
+supported, I cannot guarantee that _everything_ is supported just yet.
+
+### Core Functionality
+
+- **Syntax-aware formatting** - Deep understanding of nftables grammar with
+  semantic preservation
+- **Multi-family support** - Handles `inet`, `ip`, `ip6`, `arp`, `bridge`, and
+  `netdev` table families
+- **Flexible indentation** - Configurable tabs/spaces with custom depth
+- **CIDR notation** - Proper handling of network addresses (`192.168.1.0/24`)
+- **Chain properties** - Hooks, priorities (including negative), policies,
+  device bindings
+
+### Advanced Features
+
+- **CST** - Lossless representation preserving all tokens
+- **Debug mode** - Comprehensive inspection of lexer tokens, AST, and CST
+- **Validation** - Syntax checking with precise error locations
+- **Optimization** - Configurable empty line reduction and whitespace control
+
+## Usage
+
+```bash
+# Basic formatting
+nff -f /etc/nftables.conf
+
+# Custom indentation (4 spaces)
+nff -f config.nft --indent spaces --spaces 4
+
+# Optimize formatting (reduce empty lines)
+nff -f config.nft --optimize
+
+# Output to file
+nff -f config.nft -o formatted.nft
+
+# Syntax validation only
+nff -f config.nft --check
+
+# Debug output for development (or debugging)
+nff -f config.nft --debug
+```
+
+## Architecture
+
+### Processing Pipeline
+
+nff implements a multi-stage pipeline:
+
+```mermaid
+graph TD
+    Input --> Lexer
+    Lexer --> Tokens
+    Lexer --> Parser
+    Tokens --> Parser
+    Parser --> CST
+    Parser --> AST
+    AST --> Formatter
+    Formatter --> Output
+    CST --> Formatter
+```
+
+## Installation
+
+Recommended way of installing nff is to use Nix.
+
+## Development
+
+### Testing
+
+```bash
+# Run test suite
+cargo test
+
+# Run with verbose output
+cargo test -- --nocapture
+
+# Test specific module
+cargo test lexer
+```
+
+### Code Quality
+
+```bash
+# Check compilation
+cargo check
+
+# Format code
+cargo fmt
+
+# Lint code
+cargo clippy
+
+# Check for unused dependencies
+cargo machete
+```
+
+## Supported nftables Features
+
+### Table Families
+
+- **inet** - Dual-stack IPv4/IPv6 (most common)
+- **ip** - IPv4 only
+- **ip6** - IPv6 only
+- **arp** - ARP protocol
+- **bridge** - Bridge/Layer 2
+- **netdev** - Network device (ingress/egress)
+
+### Chain Types & Hooks
+
+- **filter**: `input`, `forward`, `output`
+- **nat**: `prerouting`, `input`, `output`, `postrouting`
+- **route**: `output`
+- **security**: `input`, `forward`, `output`
+
+### Expression Types
+
+- **Protocol matching**: `ip protocol tcp`, `tcp dport 80`
+- **Interface matching**: `iifname "eth0"`, `oifname "wlan0"`
+- **Address matching**: `ip saddr 192.168.1.0/24`, `ip6 daddr ::1`
+- **Connection tracking**: `ct state established,related`
+- **Port specifications**: `tcp dport { 22, 80, 443 }`
+- **Rate limiting**: `limit rate 10/minute burst 5 packets`
+- **Sets and maps**: Named sets with timeout support
+
+### Actions & Statements
+
+- **Verdicts**: `accept`, `drop`, `reject`, `return`
+- **NAT**: `snat to 192.168.1.1`, `dnat to 192.168.1.100:80`
+- **Marking**: `mark set 0x1`, `ct mark set 0x1`
+- **Logging**: `log prefix "dropped: "`
+- **Counters**: `counter packets 0 bytes 0`
+
+## Examples
+
+### Basic Firewall
+
+Input (minified):
+
+```nftables
+table inet firewall{chain input{type filter hook input priority 0;policy drop;ct state established,related accept;iifname lo accept;tcp dport 22 accept}}
+```
+
+Output (formatted):
+
+```nftables
+#!/usr/sbin/nft -f
+table inet firewall {
+        chain input {
+                type filter hook input priority 0; policy drop;
+                ct state established,related accept
+                iifname lo accept
+                tcp dport 22 accept
+        }
+}
+```
+
+### NAT Configuration
+
+```nftables
+#!/usr/sbin/nft -f
+table ip nat {
+        chain prerouting {
+                type nat hook prerouting priority -100; policy accept;
+                iifname "eth0" tcp dport 80 dnat to 192.168.1.100:8080
+        }
+
+        chain postrouting {
+                type nat hook postrouting priority 100; policy accept;
+                oifname "eth0" masquerade
+        }
+}
+```
+
+### Rate Limiting
+
+```nftables
+#!/usr/sbin/nft -f
+table inet protection {
+        chain input {
+                type filter hook input priority 0; policy accept;
+                tcp dport 22 limit rate 5/minute burst 10 packets accept
+                tcp dport 22 drop
+        }
+}
+```
+
+## Contributing
+
+### Code Style
+
+- Follow `cargo fmt` formatting
+- Use `cargo clippy` recommendations
+- Maintain comprehensive documentation
+- Add tests for new features
+
+### Testing Strategy (WIP)
+
+- **Unit tests**: Individual component validation
+- **Integration tests**: End-to-end formatting verification
+- **Regression tests**: Known issue prevention
+- **Performance tests**: Benchmark critical paths
+
+### Building
+
+Build with `cargo build` as usual. If you are using Nix, you will also want to
+ensure that the Nix package builds as expected.
+
+## Technical Notes
+
+### CST Implementation
+
+The Concrete Syntax Tree (CST) preserves all source information including:
+
+- Whitespace and indentation
+- Comments and their positions
+- Token-level error recovery
+- Lossless round-trip formatting
+
+### Parser Architecture
+
+Below are the design goals of nff's architechture.
+
+- **Error recovery**: Continues parsing after syntax errors
+- **Incremental parsing**: Supports partial file processing
+- **Memory efficiency**: Streaming token processing where possible
+- **Grammar completeness**: Covers full nftables syntax specification
+
+## License
+
+nff is licensed under [MPL v2.0](LICENSE). See license file for more details on
+what the license entails.