Add queueing bridges to docs

intro.rst

@@ -47,13 +47,14 @@ Client-Side Libraries
 * `davidthewatson/postgrest_python_requests_client <https://github.com/davidthewatson/postgrest_python_requests_client>`_ - Python
 * `calebmer/postgrest-client <https://github.com/calebmer/postgrest-client>`_ - JS
 
-Extensions
+External Notification
-----------
+---------------------
 
-* `diogob/postgrest-ws <https://github.com/diogob/postgrest-ws>`_ - expose web sockets for PostgreSQL's LISTEN/NOTIFY
+These are PostgreSQL bridges that propagate LISTEN/NOTIFY to external queues for further processing. This allows stored procedures to initiate actions outside the database such as sending emails.
-* `srid/spas <https://github.com/srid/spas>`_ - allow file uploads and basic auth
+
-* `svmnotn/postgrest-auth <https://github.com/svmnotn/postgrest-auth>`_ - OAuth2-inspired external auth server
+* `matthewmueller/pg-sns-bridge <https://github.com/matthewmueller/pg-sns-bridge>`_ - Amazon SNS
-* `nblumoe/postgrest-oauth <https://github.com/nblumoe/postgrest-oauth>`_ - OAuth2 WAI middleware
+* `aweber/pgsql-listen-exchange <https://github.com/aweber/pgsql-listen-exchange>`_ - RabbitMQ
+* `SpiderOak/skeeter <https://github.com/SpiderOak/skeeter>`_ - ZeroMQ
 
 Example Apps
 ------------
@@ -80,6 +81,14 @@ In Production
 * `Image-charts <https://image-charts.com/>`_
 * `Drip Depot <https://www.dripdepot.com/>`_
 
+Extensions
+----------
+
+* `diogob/postgrest-ws <https://github.com/diogob/postgrest-ws>`_ - expose web sockets for PostgreSQL's LISTEN/NOTIFY
+* `srid/spas <https://github.com/srid/spas>`_ - allow file uploads and basic auth
+* `svmnotn/postgrest-auth <https://github.com/svmnotn/postgrest-auth>`_ - OAuth2-inspired external auth server
+* `nblumoe/postgrest-oauth <https://github.com/nblumoe/postgrest-oauth>`_ - OAuth2 WAI middleware
+
 Commercial PaaS
 ---------------