[libc] Add 'Maintainers' file

Summary:
These files give people a good idea who to ask for guidance.

Author: jhuber6

libc/Maintainers.md

@@ -0,0 +1,10 @@
+# LLVM C Library Maintainers
+
+This file is a list of the
+[maintainers](https://llvm.org/docs/DeveloperPolicy.html#maintainers) for
+the LLVM C library.
+
+# Current Maintainers
+
+Joseph Huber \
+[email protected] (email), [jhuber6](https://github.com/jhuber6) (GitHub)