Make README header render larger than subheaders
Make README header render larger than subheaders

Summary:
"ReDex: An Android Bytecode Optimizer" was rendering smaller than "Quick Start Guide" on GitHub, which looks kind of weird.
Closes https://github.com/facebook/redex/pull/167

Differential Revision: D4173337

Pulled By: int3

fbshipit-source-id: 4cc4f5be982242c366584265095b4c11c6c67cf2

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex