docs: Update usage examples to use semver tags

- Change @main references to @v1 (major version tag)
- Add versioning section explaining tag options:
  - @v1 for auto-updates within major version
  - @v1.0.0 for pinned versions
  - @main for development (not recommended)

README.md

@@ -22,16 +22,23 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: https://git.johnogle.info/johno/gitea-actions/nix-setup@main
+      - uses: https://git.johnogle.info/johno/gitea-actions/nix-setup@v1
       - run: nix flake check
 ```
 
 > **Note:** Full URL required for Gitea Actions to reference actions from the same instance (short-form defaults to GitHub).
 
+**Versioning:**
+
+This repo uses semantic versioning. Reference actions by:
+- `@v1` - Major version (recommended, auto-updates to latest v1.x.x)
+- `@v1.0.0` - Specific version (pinned, no auto-updates)
+- `@main` - Latest development (not recommended for production)
+
 **With custom cache prefix:**
 
 ```yaml
-- uses: https://git.johnogle.info/johno/gitea-actions/nix-setup@main
+- uses: https://git.johnogle.info/johno/gitea-actions/nix-setup@v1
   with:
     cache-name: 'my-project-'
 ```