Add FAQ about RDS/Heroku (#357)

faq/faq.rst

@@ -86,3 +86,13 @@ I forgot the distribution column of a table, how do I find it?
 --------------------------------------------------------------
 
 The Citus coordinator node metadata tables contain this information. See :ref:`finding_dist_col`.
+
+Why does pg_relation_size report zero bytes for a distributed table?
+--------------------------------------------------------------------
+
+The data in distributed tables lives on the worker nodes (in shards), not on the coordinator. A true measure of distributed table size is obtained as a sum of shard sizes. Citus provides helper functions to query this information. See :ref:`table_size` to learn more.
+
+Can I run Citus on Heroku or Amazon RDS?
+----------------------------------------
+
+At this time Heroku and Amazon do not support running Citus directly on top of Heroku PostgreSQL or Amazon RDS. It is up to them if/when they enable the Citus extension. If you are looking for something similar, `Citus Cloud <https://www.citusdata.com/product/cloud>`_ is our database-as-a-service which we fully manage for you. It runs on top of AWS (like both RDS and Heroku PostgreSQL) and should provide a very similar product experience, with the addition of Citus' horizontal scaling.