Merge pull request brianc#796 from coderhaoxin/pg-then-link

Add link of pg-then to readme

Author: brianc

README.md

@@ -147,6 +147,7 @@ node-postgres is by design pretty light on abstractions.  These are some handy m
 - [iceddev/pg-transact](https://github.com/iceddev/pg-transact) - A nicer API on node-postgres transactions
 - [sehrope/node-pg-db](https://github.com/sehrope/node-pg-db) - Simpler interface, named parameter support, transaction management and event hooks.
 - [vitaly-t/pg-promise](https://github.com/vitaly-t/pg-promise) - Use node-postgres via [Promises/A+](https://promisesaplus.com/).
+- [pg-then](https://github.com/coderhaoxin/pg-then) A tiny wrapper of `pg` for promise api.
 
 ## License