Platform/ARM: Add Readme.md

This covers the bulk of the information originally present in
https://github.com/tianocore/tianocore.github.io/wiki/ArmPlatformPkg-AArch64


regarding building and running the Foundation/Base FVP Platforms.

The sections on fetching source have been delegated to the root Readme.md

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Nariman Poushin <nariman.poushin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>