Summary:
Release notes: We got a logo!
Updating README to include logo.
Will create an issue to update website accordingly.
Closes https://github.com/facebook/prepack/pull/2189
Differential Revision: D8725906
Pulled By: NTillmann
fbshipit-source-id: 2f9b3b1d4afbd4fc42f137fcda9e4edac390d272