Manifest: Rewrite the README

Outdated information and incomplete support should not be around,
especially not in the manifest project. The README there should be
relevant and helpful which this rewrite aims to accomplish.

Change-Id: I4703f191a96a3c256f2eed41864d345136cee1e5
Merge branch 'kitkat' of github.com:AOSPA/manifest into kitkat

Conflicts:
	readme.md