Add a blurb about installing PostgreSQL extensions

postgres/content.md

@@ -187,6 +187,12 @@ There are many ways to set PostgreSQL server configuration. For information on w
 	$ docker run -d --name some-postgres %%IMAGE%% -c 'shared_buffers=256MB' -c 'max_connections=200'
 	```
 
+## Additional Extensions
+
+When using the default (Debian-based) variants, installing additional extensions (such as PostGIS) should be as simple as installing the relevant packages (see [github.com/appropriate/docker-postgis](https://github.com/appropriate/docker-postgis/blob/f6d28e4a1871b1f72e1c893ff103f10b6d7cb6e1/10-2.4/Dockerfile) for a concrete example).
+
+When using the Alpine variants, any postgres extension not listed in [postgres-contrib](https://www.postgresql.org/docs/10/static/contrib.html) will need to be compiled in your own image (again, see [github.com/appropriate/docker-postgis](https://github.com/appropriate/docker-postgis/blob/f6d28e4a1871b1f72e1c893ff103f10b6d7cb6e1/10-2.4/alpine/Dockerfile) for a concrete example).
+
 # Caveats
 
 If there is no database when `postgres` starts in a container, then `postgres` will create the default database for you. While this is the expected behavior of `postgres`, this means that it will not accept incoming connections during that time. This may cause issues when using automation tools, such as `docker-compose`, that start several containers simultaneously.