Incomplete type list in commit guidelines: add "release" type

[marionebl]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Feature request
[x] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

The list of allowed commit types at https://github.com/angular/angular/blob/master/CONTRIBUTING.md#type currently list no type appropriate for project activities like releasing, for which previously the chore type was used.

The Angular project itself uses a release type, e.g. here: https://github.com/angular/angular/commits/master/package.json

Expected behavior

The commit conventions list all types in use. Add "release" to list of allowed types

What is the motivation / use case for changing the behavior?

Create a reliable single source of truth for commit conventions external tools such as

• https://github.com/marionebl/commitlint
• https://github.com/conventional-changelog/standard-version
• https://github.com/conventional-changelog/conventional-changelog/