Add a README and a standardizable VERSION

* Changes the compatbility matrix version to 5.0 & marks optional
* Tweaks the interface name in case it doesn't get removed soon
  (otherwise readme looks silly)
* Adds a README.md to mapper/stable-c
* Adds VTS test to validate versions match

Test: build + mapper VTS

Change-Id: I9b1256e07aaf2876d579a5487d2051c60097fb70