Add -O2 to the adding an op HOWTO (#4195)

Based on a [request from StackOverflow](http://stackoverflow.com/questions/39280669/best-way-to-modify-a-built-in-tensorflow-kernel/39301780?noredirect=1#comment65975519_39301780) where a user observed that the same code compiled following these instructions is 10x slower than code built as part of the binary installation.