Add documentation template files.

To build documents, need to the following command manually:
  msbuild docs\tools\FSharp.Data.MsgPack.Docs.proj
then documents will be generated in docs\output directory.

To build to check locally, add msbuild parameter:
  msbuild docs\tools\FSharp.Data.MsgPack.Docs.proj /p:Configuration=Debug

To rebuild documents:
  msbuild docs\tools\FSharp.Data.MsgPack.Docs.proj /t:Rebuild