Add GalagoPro3 board details to global build and documentation

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2207



 * Updates maintainers for the System 76 GalagoPro3 board
 * Updated Readme.md with System 76 GalagoPro3 board details
 * Adds the ability to build the GalagoPro3 board to build.cfg

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Ankit Sinha <ankit.sinha@intel.com>
Cc: Jeremy Soller <jeremy@system76.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Ankit Sinha <ankit.sinha@intel.com>
Reviewed-by: Jeremy Soller <jeremy@system76.com>