Provide specific scopes in contributing guidelines

[comewalk]

This repository runs tests on Travis CI and AppVeyor. Although checking commit message, no one provides specific scopes in CONTRIBUTING.md.

Scope

The scope could be anything specifying place of the commit change. For example
Compiler, ElementInjector, etc.

Actually validate-commit-message.js checks scopes. If someone write wrong scopes in commit message, Travis CI and AppVeyor will fail. (me, too. at build #6536)

It's about time to provide specific scopes in CONTRIBUTING.md.
It looks like below.

### Scope
The scope could be anything specifying place of the commit change.
* @angular/cli
* @ngtools/json-schema
* @ngtools/logger
* @ngtools/webpack
* packaging
* changelog

Any thought?

[clydin]

A documentation update is in progress.

[comewalk]

@clydin sounds good 👍

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