docs: Document how to use a self-modified dependency with Zulip in dev.

As explained in the doc, sometimes you want to test your changes to a
dependency WITH Zulip without waiting for upstream merge and release.

requirements/README.md

@@ -18,3 +18,32 @@ Steps to update a lock file, e.g. to update ipython from 5.3.0 to latest version
 2. Increase `PROVISION_VERSION` in `version.py`.
 3. Run `./tools/provision` to install the new deps and test them.
 4. Commit your changes.
+
+## Testing custom modifications of a dependency
+
+When working on Zulip, sometimes it is necessary to also make
+modifications to a dependency (either to add a feature that we will
+rely on, or to fix a bug that needs to be fixed for the intended
+changes to work in Zulip) - with the idea that eventually they will be
+merged upstream and included in an official release.
+
+That process can take time however, and sometimes it'd be good to be
+able to create and test your Zulip changes on top of changes to the
+upstream package, without waiting for the potentially lengthy code
+review and release process of the upstream dependency.
+
+You can do this forking the upstream project, making the changes on a
+branch in your fork, and then replacing the package's entry in
+`dev.in` or `prod.in` with an appropriate GitHub link to the branch
+with your modifications. The files have various examples of how this
+should be done, but essentially you will add an entry looking like
+this:
+
+```
+https://github.com/<your GitHub>/<package name>/archive/<commit hash>.zip#egg=<package name>==<version>+git
+```
+
+After that, you can follow the above process involving
+`./tools/update-locked-requirements` and the following steps to have
+the modified package installed in your dev environment, where it can
+be used for testing.