Docs: Specify commit message format

Add initial `CONTRIBUTING.md` specifying the commit message format to
use.

CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# Contributing
+
+## Commit Message Format
+
+The commit message format is as follows:
+
+```
+Tag: Short description
+
+Longer description here if necessary
+```
+
+The `Tag` should be one of the following:
+
+- `Fix` - for a bug fix.
+- `Update` - for a backwards compatible enhancement.
+- `Feature` - for a new feature.
+- `Breaking` - for a backwards-incompatible enhancement or feature.
+- `Perf` - for a code change that improves performance.
+- `Refactor` - for a code change that neither fixes a bug nor adds a feature (nor is `Perf`).
+- `Build` - for changes that affect the build system or external dependencies.
+- `Test` - for adding or correcting tests.
+- `Docs` - for changes to documentation only.
+- `Other` - for anything that isn't covered by the tags above.
+
+Those tags are an adapted version of the tags eslint (<https://eslint.org/docs/developer-guide/contributing/pull-requests#step-2-make-your-changes>) and of the tags Angular (<https://github.com/angular/angular/blob/master/CONTRIBUTING.md#type>) uses.