Move README.md to compiler-builtins, CONTRIBUTING.md to root

Do this in a commit so git tracks the move; a new README will be
introduced in the root, and CONTRIBUTING will be updated to apply to
both crates.

Author: tgross35

libm/CONTRIBUTING.md CONTRIBUTING.mdlibm/CONTRIBUTING.md renamed to CONTRIBUTING.md

@@ -18,8 +18,8 @@ include = [
   '/configure.rs',
   '/src/*',
   '../LICENSE.txt',
-  '../README.md',
   '../compiler-rt/*',
+  'README.md',
   'libm/src/math/*',
 ]
 links = 'compiler-rt'