added building wp8 shaders instructions

build/winrt/wp8_precompiled_shaders.txt

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+How to compile precompiled shaders for Windows Phone 8.0 (WP8)
+
+1. Open the VS project Cocos2dShaderCompiler\Cocos2dShaderCompiler.sln
+2. Build and run the project
+3. Click the Compile button
+4. Select the file cocos\platform\wp8\shaders\precompiledshaders.h in the file dialog box
+5. Program will precompile all of the cached cocos2d-x shaders.