FAQ entries about extensions (#358)

* FAQ entries about extensions

* Emphasize that pg_shard is no more

faq/faq.rst

@@ -101,3 +101,15 @@ Can I shard by schema on Citus for multi-tenant applications?
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 It turns out that while storing each tenant's information in a separate schema can be an attractive way to start when dealing with tenants, it leads to problems down the road. In Citus we partition by the tenant_id, and a shard can contain data from several tenants. To learn more about the reason for this design, see our article `Lessons learned from PostgreSQL schema sharding <https://www.citusdata.com/blog/2016/12/18/schema-sharding-lessons/>`_.
+
+How does cstore_fdw work with Citus?
+------------------------------------
+
+Citus treats cstore_fdw tables just like regular PostgreSQL tables. When cstore_fdw is used with Citus, each logical shard is created as a foreign cstore_fdw table instead of a regular PostgreSQL table. If your cstore_fdw use case is suitable for the distributed nature of Citus (e.g. large dataset archival and reporting), the two can be used to provide a powerful tool which combines query parallelization, seamless sharding and HA benefits of Citus with superior compression and I/O utilization of cstore_fdw.
+
+What happened to pg_shard?
+--------------------------
+
+The pg_shard extension is deprecated and no longer supported.
+
+Starting with the open-source release of Citus v5.x, pg_shard's codebase has been merged into Citus to offer you a unified solution which provides the advanced distributed query planning previously only enjoyed by CitusDB customers while preserving the simple and transparent sharding and real-time writes and reads pg_shard brought to the PostgreSQL ecosystem. Our flagship product, Citus, provides a superset of the functionality of pg_shard and we have migration steps to help existing users to perform a drop-in replacement. Please `contact us <https://www.citusdata.com/about/contact_us>`_ for more information.