Add info on the dependency into the raftable README.

Author: kvap

contrib/raftable/README

@@ -1,6 +1,17 @@
+Raftable
+========
+
 This extension allows you to have a key-value table replicated between several
 Postgres instances over Raft protocol.
 
+Depends on
+----------
+
+Raft implementation in C: https://github.com/kvap/raft
+
+Internals
+---------
+
 Each 'postgres' instance starts a background worker 'raftable' and creates a
 shared memory segment for storing the 'state'. All 'raftable' workers are
 communicating Raft protocol over UDP.