Update description for Linux support
Update description for Linux support

Added Linux-specific information:
- about XCB
- requirements
- how to simulate media keys.

This is an updated PR version of #97, which was accidentally pushed to mac branch.
gmelikov/browser-media-keys,carlin-q-scott/browser-media-keys,carlin-q-scott/browser-media-keys,gmelikov/browser-media-keys,gmelikov/browser-media-keys