Adding information about updating developer environment to the developer docs

doc/developer.md

@@ -64,12 +64,18 @@ tox -e dev
 . .tox/dev/bin/activate
 ```
 
-Finally, ou can also explicitly install all the Python dependencies for sourmash by running
+Finally, you can also explicitly install all the Python dependencies for sourmash by running
 ```
 pip install -r requirements.txt
 ```
 (but they are already installed in the virtualenv created with `tox -e dev`).
 
+## Updating your developer environment
+
+To update rust to the latest version, use `rustup update`.
+
+To update your Python dependencies to the latest required for sourmash, you can run `pip install -r requirements.txt`.
+
 ## Running tests and checks
 
 We use [`tox`](https://tox.readthedocs.io) for managing dependencies and