Move lib/Fuzzer docs from a README.txt to a proper .rst file.

Summary:
Move lib/Fuzzer docs from a README.txt to a proper .rst file.
This change does not add any content, just formatting.

Test Plan: n/a

Reviewers: samsonov

Reviewed By: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8710

llvm-svn: 233638