chore(CONTRIBUTING): merge relevant updates from Angular.js

Port changes to Angular.js CONTRIBUTING.md since AngularDart version
was forked.

Closes #748
fix(animation): correct broken build http://dartbug.com/17634
fix(scope): allow concurrent fire/add/remove on listeners
feat(Scope): Improve ScopeStats reporting

Closes #744
fix(forms): ensure models are validated when validator attributes change
fix(NgModelValidators): ensure all validators can properly toggle attribute values
chore(forms): cleanup unused code

Closes #761
docs(EventHandler): Add EventHandler documentation.

Add documentation to EventHandler. This change also makes fields and methods within EventHandler, which are not intended for public use, private.

Closes #764
fix(component): revert regression of injecting Element/Node into Component
feat(filters): revert filter being restricted to top level

See: 5bcea6492f6d0fd39ba316fa3b241c50bb94de8d
feat(doc): Library description for angular.animate
feat(doc): Animation library documentation updates and fixes.

Closes #760
test(symbols): Track the symbols we export from the angular library

Closes #398