edk2: Add Readme.md to root of edk2 repository

edk2: Add Readme.md to root of edk2 repository

https://bugzilla.tianocore.org/show_bug.cgi?id=643

Add Readme.md with a brief description of the EDK II
open source project along with links to contribution
agreement, licenses, and resources.

Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Andrew Fish <afish@apple.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>