Add note about updating CHANGELOG

Add a section about updating the CHANGELOG.md with information about changes when submitting a PR.

CONTRIBUTING.md

@@ -75,6 +75,11 @@ like so:
 Resolves: #123
 ```
 
+## Don't forget to update the `CHANGELOG.md`
+
+Don't forget to update the `CHANGELOG.md` file with your change in the
+`Unreleased` section
+
 ## Squash commits and rebase before merging
 
 When you are ready to merge then squash your commits so that they form a
@@ -90,4 +95,4 @@ This project makes use of the following tools to lint the lua and .proto files
 
 It is not mandatory to run these yourself on your local machine however they
 are run as part of the automated checks when you create a pull request so it
-may save you time to run them yourself before-hand.
+may save you time to run them yourself before-hand.