Add a new section about making updates

There are some useful principles when adding new branches with new
features to this project. This section covers what you need to think
about.

Author: Mike Mindel

README.md

@@ -154,6 +154,14 @@ Relative imports not only leave you free to rename your package later without ch
 
 Now delete all the docs that you've just followed, and write something suitable for your new project!
 
-```
+## Add updates to this project
 
-```
+- Create a new branch for each new feature you wish to add e.g. fly.io if, for example, you're adding a branch and specific code to roll out to the fly.io cloud service.
+
+- Only add the **new** modules you are interested in. There's no need to keep the modules which are already in `requirements.in` and `requirements-dev.in` in the `main` branch. Only add the new modules (if any) which are relevant to that branch.
+
+- The same thing applies to this README.md file. You don't need this text as it only applies to the `main` branch. Only add the relevant section to the README.md for the new branch, otherwise if you make a change to this file in `main`, you will need to sync the changes to the new branch too.
+
+- There's no need to check in `requirements.txt` or `requirements-dev.txt` as they can be generated on the fly with `make update` from the Makefile.
+
+For future research: Is there a way to only add the parts we are interested in without a merge conflict?