docs: document project motivation, usage and scanner development

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ic762bb0d4b2fa619907a9e4f94278b5f6a6a6964

docs/SCANNERS.md

@@ -0,0 +1,60 @@
+# Creating a Custom Scanner
+
+pscand comes with four scanners built-in, but you may easily create your own
+scanners for future extensibility. The process is simple.
+
+1. Create your own crate
+2. Implement the `Scanner` trait
+3. Build, and place it into a scanner directory
+
+See below:
+
+## Creating your scanner crate
+
+```toml
+# scanners/scanner-custom/Cargo.toml
+[package]
+name = "scanner-custom"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+pscand-core = { workspace = true }
+```
+
+## Implementing the `Scanner` trait
+
+```rust
+use pscand_core::scanner::Scanner;
+use pscand_macros::scanner;
+
+pub struct CustomScanner;
+
+impl Scanner for CustomScanner {
+    fn name(&self) -> &str {
+        "custom"
+    }
+
+    fn collect(&self) -> Result<serde_json::Value, Box<dyn std::error::Error>> {
+        // Collect your metrics
+        Ok(serde_json::json!({
+            "value": 42
+        }))
+    }
+}
+
+#[scanner]
+static SCANNER: CustomScanner = CustomScanner;
+```
+
+## Building and installing
+
+```bash
+cargo build --release
+# Install to a directory in PSCAND_SCANNER_DIRS (e.g., ~/.local/share/pscand/scanners)
+install -Dm755 target/release/libscanner_custom.so \
+    ~/.local/share/pscand/scanners/scanner-custom.so
+```