docs: add commit message convention to CONTRIBUTING.md

Author: treuherz

CONTRIBUTING.md

@@ -60,6 +60,23 @@ go test -fuzz=FuzzBooleanEvaluation ./e2e/evaluation_fuzz_test.go
 ```
 substituting the name of the fuzz as appropriate.
 
+### Commit messages
+
+We require commit messages and PR titles to follow the [Conventional Commits specification](https://www.conventionalcommits.org/en/v1.0.0/), meaning all titles should begin with a specifier for the type of change being made, followed by a colon, like `feat: add support for boolean flags` or `perf: improve flag evaluation times by removing time.Sleep`.
+
+The full list of available types is:
+ - `feat`: A new feature
+ - `fix`: A bug fix
+ - `docs`: Documentation only changes
+ - `style`: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
+ - `refactor`: A code change that neither fixes a bug nor adds a feature
+ - `perf`: A code change that improves performance
+ - `test`: Adding missing tests or correcting existing tests
+ - `build`: Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm)
+ - `ci`: Changes to our CI configuration files and scripts (example scopes: Travis, Circle, BrowserStack, SauceLabs)
+ - `chore`: Other changes that don't modify src or test files
+ - `revert`: Reverts a previous commit
+
 ### Releases
 
 This repo uses Release Please to release packages. Release Please set up a running PR that tracks all changes for the library components, and maintains the versions according to conventional commits, generated when PRs are merged.