COMMON: make XMLParser::loadStream() fail when stream is null

Some users of this method relies on it to fail when
an invalid stream is passed to it
(E.g. VirtualKeyboard::openPack).
DOCS: Add CONTRIBUTING.md

This will be displayed on github as "guidelines for contributing".
Merge pull request #534 from skristiansson/pr/loadstream

COMMON: make XMLParser::loadStream() fail when stream is null
COMMON: Remove executable bit
Merge https://github.com/scummvm/scummvm