Document how to run Synapse (#15022)

* Document how to run Synapse

* Changelog for 15022

* Update docs/development/contributing_guide.md

changelog.d/15022.doc

@@ -0,0 +1 @@
+Document how to start Synapse in the contributing guide.

docs/development/contributing_guide.md

@@ -78,6 +78,19 @@ poetry install --extras all
 
 This will install the runtime and developer dependencies for the project.
 
+## Running Synapse via poetry
+
+To start a local instance of Synapse in the locked poetry environment, create a config file:
+
+```sh
+cp docs/sample_config.yaml homeserver.yaml
+```
+
+Now edit homeserver.yaml, and run Synapse with:
+
+```sh
+poetry run python -m synapse.app.homeserver -c homeserver.yaml
+```
 
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