Adding Google Play Games C++ SDK instructions.
Adding Google Play Games C++ SDK instructions.

The SDK needs to copied into the dependencies directory, so adding
instructions on how to do so.

Tested on Linux.

Change-Id: I42fcb02af8e92ed8c373dc7e282708c9220900ad

google/zooshi,google/zooshi,google/zooshi,google/zooshi,google/zooshi