FAQ entry for backend support (#3282)

so that I can close:
https://github.com/apache/incubator-superset/pull/1470/files

docs/faq.rst

@@ -171,3 +171,16 @@ How do I go about developing a new visualization type?
 Here's an example as a Github PR with comments that describe what the
 different sections of the code do:
 https://github.com/airbnb/superset/pull/3013
+
+What database engine can I use as a backend for Superset?
+--------------------------------------------------------
+
+To clarify, the *database backend* is an OLTP database used by Superset to store its internal
+information like your list of users, slices and dashboard definitions.
+
+Superset is tested using Mysql, Postgresql and Sqlite for its backend. It's recommended you
+install Superset on one of these database server for production.
+
+Using a column-store, non-OLTP databases like Vertica, Redshift or Presto as a database backend simply won't work as these databases are not designed for this type of workload. Installation on Oracle, Microsoft SQL Server, or other OLTP databases may work but isn't tested.
+
+Please note that pretty much any databases that have a SqlAlchemy integration should work perfectly fine as a datasource for Superset, just not as the OLTP backend.