docs(*): update CONTRIBUTING.md and create DEVELOPERS.md

CONTRIBUTING.md
- focus on basic info about issues and pull requests for new contributors
- move development heavy info to DEVELOPERS.md + add links
- remove outdated info

DEVELOPERS.md
- contains info about project setup, coding rules, and commit message guidelines from CONTRIBUTING.md
- add and update info about writing docs from Wiki
- add info about development setup from docs contribute.md
- add info about running tests on Saucelabs / Browserstack

Closes #7303
Closes #9444 
Closes #16297

README.md

@@ -16,35 +16,27 @@ piece of cake. Best of all? It makes development fun!
 * Tutorial: https://docs.angularjs.org/tutorial
 * API Docs: https://docs.angularjs.org/api
 * Developer Guide: https://docs.angularjs.org/guide
-* Contribution guidelines: [CONTRIBUTING.md](https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md)
+* Contribution guidelines: [CONTRIBUTING.md](CONTRIBUTING.md)
+* Core Development: [DEVELOPERS.md](DEVELOPERS.md)
 * Dashboard: https://dashboard.angularjs.org
 
 ##### Looking for the new Angular? Go here: https://github.com/angular/angular
 
-Building AngularJS
----------
-[Once you have set up your environment](https://docs.angularjs.org/misc/contribute), just run:
+Documentation
+--------------------
+Go to https://docs.angularjs.org
 
-    grunt package
-
-
-Running tests
--------------
-To execute all unit tests, use:
-
-    grunt test:unit
-
-To execute end-to-end (e2e) tests, use:
-
-    grunt package
-    grunt test:e2e
-
-To learn more about the grunt tasks, run `grunt --help`
-
-Contribute & Develop
+Contribute
 --------------------
 
-We've set up a separate document for our [contribution guidelines](https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md).
+We've set up a separate document for our
+[contribution guidelines](https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md).
+
+Develop
+--------------------
+
+We've set up a separate document for
+[developers](https://github.com/angular/angular.js/blob/master/DEVELOPERS.md).
 
 
 [![Analytics](https://ga-beacon.appspot.com/UA-8594346-11/angular.js/README.md?pixel)](https://github.com/igrigorik/ga-beacon)