Add explanation of available types for Conventional Commits (#3868)

Author: marecabo

.github/pull_request_template.md

@@ -3,6 +3,19 @@ We are using [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.
 #### Title
 `<type>[optional scope]: <description>` e.g.: fix(QSim): Correctly calculate travel times on links
 
+Available types:
+ - 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
+
 #### Body
 Optionally, describe the change in more detail.