Fix broken link in CONTRIBUTING.md

The link titled "Build and share plugins" goes to docs/plugins.md, which
doesn't exist. It should go to docs/guides/plugins.md
Merge pull request #1489 from walsh9/feature/doc-link-fix

Fix broken link in CONTRIBUTING.md
fixes #1512, fixes #1507

On ended, pause player if not paused or looping
IE11 (and maybe other browsers as well) only fire 'ended' event when the
video ends and doesn't fire pause. This makes IE11 not reset it's state
into a paused state that allows a user to replay immediately. However, a
two clicks on the play/pause button will allow you to replay.
In Chrome, we get first a pause event and then an ended event. When
'loop' is set, neither the last pause nor ended fire.
In the flash tech, the pause and ended events fire like in chrome in all
browsers.

Add tests for onEnded change

Appease jshint.
Added line to the changelog for #1512
Release 4.8.2
Merge branch 'stable'

Conflicts:
	dist/video-js/video-js.css
	dist/video-js/video-js.min.css
	dist/video-js/video.dev.js
	dist/video-js/video.js
Updated contrib.json to comply with new contrib formatting