Mention python3-venv and libpq-dev dependencies in contribution guide (#11740)

changelog.d/11740.doc

+Mention python3-venv and libpq-dev dependencies in contribution guide.

docs/development/contributing_guide.md

@@ -20,7 +20,9 @@ recommended for development. More information about WSL can be found at
 <https://docs.microsoft.com/en-us/windows/wsl/install>. Running Synapse natively
 on Windows is not officially supported.
 
-The code of Synapse is written in Python 3. To do pretty much anything, you'll need [a recent version of Python 3](https://wiki.python.org/moin/BeginnersGuide/Download).
+The code of Synapse is written in Python 3. To do pretty much anything, you'll need [a recent version of Python 3](https://www.python.org/downloads/). Your Python also needs support for [virtual environments](https://docs.python.org/3/library/venv.html). This is usually built-in, but some Linux distributions like Debian and Ubuntu split it out into its own package. Running `sudo apt install python3-venv` should be enough.
+
+Synapse can connect to PostgreSQL via the [psycopg2](https://pypi.org/project/psycopg2/) Python library. Building this library from source requires access to PostgreSQL's C header files. On Debian or Ubuntu Linux, these can be installed with `sudo apt install libpq-dev`.
 
 The source code of Synapse is hosted on GitHub. You will also need [a recent version of git](https://github.com/git-guides/install-git).