Add "Configuration"

Configuration.md

@@ -0,0 +1,88 @@
+# Configuration
+
+go-forge resolves its Forgejo URL and API token from multiple sources with a clear priority chain.
+
+## Priority Order
+
+```
+1. Flags (highest)     — passed directly to NewForgeFromConfig or ResolveConfig
+2. Environment variables — FORGE_URL, FORGE_TOKEN
+3. Defaults (lowest)   — http://localhost:3000 for URL, no token default
+```
+
+## Environment Variables
+
+| Variable      | Description                          | Default                  |
+|---------------|--------------------------------------|--------------------------|
+| `FORGE_URL`   | Base URL of the Forgejo instance     | `http://localhost:3000`  |
+| `FORGE_TOKEN` | API token for authentication         | _(none — required)_      |
+
+## Using NewForgeFromConfig
+
+The simplest way to create a configured client:
+
+```go
+// Reads from FORGE_URL and FORGE_TOKEN environment variables.
+f, err := forge.NewForgeFromConfig("", "")
+if err != nil {
+    log.Fatal(err) // "no API token configured" if FORGE_TOKEN is unset
+}
+```
+
+With flag overrides:
+
+```go
+// Flag values take priority over env vars.
+f, err := forge.NewForgeFromConfig("https://forge.example.com", "my-token")
+```
+
+## Using ResolveConfig Directly
+
+For more control, resolve the config separately:
+
+```go
+url, token, err := forge.ResolveConfig(flagURL, flagToken)
+if err != nil {
+    log.Fatal(err)
+}
+// Use url and token as needed...
+f := forge.NewForge(url, token)
+```
+
+## Using NewForge Directly
+
+If you already have the URL and token:
+
+```go
+f := forge.NewForge("https://forge.lthn.ai", "my-api-token")
+```
+
+## Client Options
+
+`NewForge` and `NewForgeFromConfig` accept variadic options:
+
+```go
+// Custom HTTP client (e.g. with timeouts, proxies, TLS config)
+f := forge.NewForge(url, token,
+    forge.WithHTTPClient(&http.Client{Timeout: 30 * time.Second}),
+)
+
+// Custom User-Agent header
+f := forge.NewForge(url, token,
+    forge.WithUserAgent("my-app/1.0"),
+)
+```
+
+| Option             | Description                   | Default          |
+|--------------------|-------------------------------|------------------|
+| `WithHTTPClient`   | Set a custom `*http.Client`   | `http.DefaultClient` |
+| `WithUserAgent`    | Set the `User-Agent` header   | `go-forge/0.1`  |
+
+## Accessing the Low-Level Client
+
+If you need direct HTTP access for custom endpoints:
+
+```go
+client := f.Client() // returns *forge.Client
+err := client.Get(ctx, "/api/v1/custom/endpoint", &result)
+```