Update implementations list to include coalaip-http-api

Author: sohkai

IMPLEMENTATIONS.md

@@ -3,7 +3,11 @@
 We're currently in the process of implementing reference implementations of this specification in
 Python. To learn more, visit our repositories for:
 
-- [`pycoalaip` - A Python library for creating COALA IP models in JSON-LD and IPLD](https://github.com/bigchaindb/pycoalaip)
-- [`pycoalaip-bigchaindb` - A plugin for `pycoalaip` to persist COALA IP models on BigchainDB](https://github.com/bigchaindb/pycoalaip-bigchaindb)
+- [`pycoalaip`](https://github.com/bigchaindb/pycoalaip) - A Python library for creating COALA IP
+  models in JSON-LD and IPLD
+    - [`pycoalaip-bigchaindb`](https://github.com/bigchaindb/pycoalaip-bigchaindb) - A plugin for
+      `pycoalaip` to persist COALA IP models on BigchainDB
+- [`coalaip-http-api`](https://github.com/bigchaindb/coalaip-http-api) - A HTTP REST API wrapper
+  around `pycoalaip`
 
 We'd love to see other other implementations! See [here](CONTRIBUTING.md) to learn how to help.