Readme.md: Add initial content

Readme.md: Add initial content

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3812

Adds the following content to Readme.md:

1. A basic explanation of how the package is organized and where to
   find important items.

2. Adds instructions for building PrmPkg with edk2.

3. Adds a "Build Flags" section to the build instructions to explain
   any build flags that may be passed to influence the build.

   The final package will not have any build flags. A reasonable
   number are temporarily used to test different flows until final
   decisions are made as to which flow to keep.

   Most notably, only Visual Studio tool chains are currently
   supported due to the modifications made to support export tables.

Cc: Andrew Fish <afish@apple.com>
Cc: Kang Gao <kang.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Michael Kubacki <michael.kubacki@microsoft.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Benjamin You <benjamin.you@intel.com>
Cc: Liu Yun <yun.y.liu@intel.com>
Cc: Ankit Sinha <ankit.sinha@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Liming Gao <gaoliming@byosoft.com.cn>
Acked-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Ankit Sinha <ankit.sinha@intel.com>