update changelog for 5.1.0

Author: robbecker-wf

CHANGELOG.md

@@ -3,6 +3,7 @@
  - `Repository`: added `updatedAt` and `license` fields.
  - Require at least Dart `2.3.0`.
  - Bump version constraint on `json_annotation`
+ - Add contents_url to PullRequestFile https://github.com/DirectMyFile/github.dart/pull/159
 
 ## v5.0.2
   - Fixed pollPublicEventsReceivedByUser to use the correct API URL https://github.com/DirectMyFile/github.dart/pull/150