Add committers file

Add a CODEOWNERS file to the project.

- Andrew for `jsonrpc` because its his crate.
- Jamil for `types` and `client` because he wrote much of the code it
  `types`.
- Ricardo for `node` because its his crate.
- Leo for `bitreq` because he offered to help maintain in at the
  Rust Bitcoin Summit.

This PR will need explicit acks from each of these devs please.

Author: tcharding

CODEOWNERS

@@ -0,0 +1,10 @@
+# Default owners for everything in the repo
+*       @tcharding
+
+# Owners for specific directories
+/bitreq/        @jamillambert @oleonardolima
+/client/        @jamillambert
+/jsonrpc/       @apoelstra
+/node/          @RCasatta
+/types/         @jamillambert
+