Manifest: Add info about syncing specific projects

Quick information about syncing individual projects should be in
the readme of the manifest as it can be helpful when the person
just wants to have a specific project up to date instead of wanting
to wait for all the projects to sync up.

Change-Id: I99383b3855b10ac31905feb3407bb74f7093c585
Merge branch 'kitkat' of github.com:AOSPA/manifest into kitkat

Conflicts:
	readme.md