Flesh out documentation for running SyTest against Synapse, including use of Postgres and worker mode. (#12271)

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

changelog.d/12271.doc

@@ -0,0 +1 @@
+Clarify documentation for running SyTest against Synapse, including use of Postgres and worker mode.

docs/development/contributing_guide.md

@@ -254,8 +254,14 @@ configuration:
 ```sh
 $ docker run --rm -it -v /path/where/you/have/cloned/the/repository\:/src:ro -v /path/to/where/you/want/logs\:/logs matrixdotorg/sytest-synapse:buster
 ```
+(Note that the paths must be full paths! You could also write `$(realpath relative/path)` if needed.)
 
-This configuration should generally cover  your needs. For more details about other configurations, see [documentation in the SyTest repo](https://github.com/matrix-org/sytest/blob/develop/docker/README.md).
+This configuration should generally cover your needs.
+
+- To run with Postgres, supply the `-e POSTGRES=1 -e MULTI_POSTGRES=1` environment flags.
+- To run with Synapse in worker mode, supply the `-e WORKERS=1 -e REDIS=1` environment flags (in addition to the Postgres flags).
+
+For more details about other configurations, see the [Docker-specific documentation in the SyTest repo](https://github.com/matrix-org/sytest/blob/develop/docker/README.md).
 
 
 ## Run the integration tests ([Complement](https://github.com/matrix-org/complement)).