docs: Add GitHub Packages installation instructions to README

Added alternative installation method via GitHub Packages for users
who need access before the npm public registry package is available.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: fergusbisset

README.md

@@ -8,6 +8,30 @@ The NHS FDP Design System provides a complete set of reusable components, design
 
 ## Installation
 
+### From npm (public registry)
+
+```bash
+npm install @nhsdigital/fdp-design-system
+```
+
+### From GitHub Packages
+
+If the npm package is not yet available, you can install from GitHub Packages:
+
+1. Create or edit `.npmrc` in your project root:
+
+```
+@nhsdigital:registry=https://npm.pkg.github.com
+```
+
+2. Authenticate with GitHub Packages (requires a GitHub PAT with `read:packages` scope):
+
+```bash
+npm login --registry=https://npm.pkg.github.com
+```
+
+3. Install the package:
+
 ```bash
 npm install @nhsdigital/fdp-design-system
 ```