Update changelog

Author: lfdebrux

CHANGELOG.md

@@ -2,6 +2,23 @@
 
 ## Unreleased
 
+### New features
+
+You can now use relative links with your site.
+
+To your `config.rb` file, add the following lines:
+
+```
+set :relative_links, true
+activate :relative_assets
+```
+
+This may help with deploying to GitHub pages.
+
+- [#291: Support sites deployed on paths other than "/" (by generating relative links)](https://github.com/alphagov/tech-docs-gem/pull/291)
+
+Thanks [@eddgrant](https://github.com/eddgrant) for contributing this feature and the associated fixes!
+
 ## 3.1.0
 
 ### New features