Update README.md and MPC source URL root

[evansgp]

I noticed that one of source URLs is now outdated. I've also updated the README.md with a couple of minor things.

Build demonstrated here: https://github.com/evansgp/docker-arduino-toolchain.

This is in relation to this thread: https://groups.google.com/a/arduino.cc/forum/#!topic/developers/6anSTArSFRY

[matthijskooijman]

I suspect that the staging branch is the most recent branch at this moment, could be that it's used in the Arduino 1.9 beta build or something. @facchinm, can you confirm this? Also, when is this beta mess going to end? :-p

[facchinm]

@matthijskooijman @evansgp I can confirm that next version of the toolchain will be based on the (cleaned-up) staging branch, so that one should be used as starting point for any development (see #47)

[evansgp]

Woops, cheers!