Make change to CHANGELOG doc

Author: Lev Gorbunov

CHANGELOG.md

@@ -7,6 +7,16 @@
 NB: [Universal Analytics is now gone](https://support.google.com/analytics/answer/11583528?hl=en) so anyone still using the older approach is likely facing broken analytics, or has developed a workaround.
 We'll return at a later date with a major breaking change that deprecates UA code.
 
+## 5.1.0
+
+### New features
+
+- [Add GA4 support](https://github.com/alphagov/tech-docs-gem/pull/350)
+
+### Fixes
+
+- [Fix syntax highlighting by pinning middleman-syntax to v3.6+](https://github.com/alphagov/tech-docs-gem/pull/427)
+
 ## 5.0.2
 
 ### Fixes