Add Contributing section to README

Author: kenstir

README.md

@@ -1,5 +1,19 @@
 # hemlock-docs
 
+## Intro
+
 Here be documentation for the Hemlock mobile apps, and the Evergreen users and administrators that support them.
 
 These docs are published to https://kenstir.github.io/hemlock-docs/ from the `main` branch and we built using [Docusaurus](https://docusaurus.io/).
+
+## Contributing
+
+1. Preview the site locally
+   ```bash
+   npm ci
+   npm start
+   ```
+1. Build and check for broken links
+   ```bash
+   npm run build
+   ```