Add instructions for xblock development

As per
https://discuss.overhang.io/t/best-practice-xblock-development-in-tutor-dev-mode/345

docs/dev.rst

@@ -108,6 +108,27 @@ To debug a local edx-platform repository, add a ``import ipdb; ipdb.set_trace()`
 
     tutor dev runserver -v /path/to/edx-platform:/openedx/edx-platform lms
 
+XBlock and edx-platform plugin development
+------------------------------------------
+
+In some cases you will have to develop features for packages that are pip-installed next to edx-platform. This is quite easy with Tutor. Just add your packages to the `$(tutor config printroot)/env/build/openedx/requirements/private.txt`` file. To avoid re-building the openedx Docker image at every change, you should add your package in editable mode. For instance::
+
+    echo "-e ./mypackage" >> "$(tutor config printroot)/env/build/openedx/requirements/private.txt"
+
+The ``requirements`` folder should have the following content::
+
+    env/build/openedx/requirements/
+        private.txt
+        mypackage/
+            setup.py
+            ...
+
+You will have to re-build the openedx Docker image once::
+    
+    tutor images build openedx
+
+You should then run the development server as usual, with ``runserver``. Every change made to the ``mypackage`` folder will be picked up and the development server will be automatically reloaded.
+
 .. _theming:
 
 Customised themes

docs/troubleshooting.rst

@@ -47,7 +47,7 @@ This is not an error with Tutor, but with your Docker installation. This is freq
     
     docker run --rm hello-world
     
-If the above command does not work, you should fix your Docker installation. Some people will suggest to run Docker as root, or with `sudo`; *do not do this*. Instead, what you should probably do is to add your user to the "docker" group. For more information, check out the `official Docker installation instructions <https://docs.docker.com/install/linux/linux-postinstall/#manage-docker-as-a-non-root-user>`__.
+If the above command does not work, you should fix your Docker installation. Some people will suggest to run Docker as root, or with ``sudo``; **do not do this**. Instead, what you should probably do is to add your user to the "docker" group. For more information, check out the `official Docker installation instructions <https://docs.docker.com/install/linux/linux-postinstall/#manage-docker-as-a-non-root-user>`__.
 
 Help! The Docker containers are eating all my RAM/CPU/CHEESE
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