Add description to README

Author: sfackler

README.md

@@ -1,6 +1,8 @@
 r2d2-postgres
 =============
 
+[rust-postgres](https://github.com/sfackler/rust-postgres) support library for the [r2d2](https://github.com/sfackler/r2d2) connection pool.
+
 Documentation is available at https://sfackler.github.io/doc/r2d2_postgres
 
 # Example