add readme.txt for UnixPkg

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@8761 6f19259b-4bc3-4df7-8a09-765794883524

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+Unix Simulation Platform
+========================
+
+UnixPkg is one of platform package which can be built to a platform's firmware from UnixPkg.dsc file.
+And this package provide a simulation platform under *INUX environment in IA32 architecture.
+
+- Same points between real platform and simulation platform:
+  1) Unix simulation platform also run with firmware device image built from UnixPkg
+  2) The working flow of simulation platform also contains SEC/PEI/DXE phase.
+
+- Different points between real platform and simulation platform:
+  1) The SEC phase in simulation platform in fact is a *INUX native application which can be run from *INUX shell;
+  2) The device in simulation platform is not real hardware but simulation component in API level;
+  3) The ThunkBus driver in simulation platform will get virtual device's desription from PCD and create virtual Device
+
+Build
+=====
+UnixPkg is built with following command:
+  build -p UnixPkg/UnixPkg.dsc -a IA32 -t ELFGCC
+  Notes: ELFGCC is defined in <Workspace>/Conf/tools_def.txt file. This tool chain use native gcc/binutil instead of 
+         cross-compiler like UNIXGCC tool chain.
+
+
+FAQ
+===
+1, I fail to build UnixPkg due to "X11/extensions/XShm.h: No such file or directory"?
+   The display adapter in UnixPkg is a virtual device written in X11 API. The library of x11proto-xext-dev is required
+   for building.
+
+2, I fail to build UnixPkg due to "/usr/bin/ld: cannot find -lXext"?   
+   libxext-dev library is required for building.
+