updated build instructions

Author: trevorcampbell

README.md

@@ -9,17 +9,36 @@ This textbook is offered under
 the [Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) License](https://creativecommons.org/licenses/by-nc-sa/4.0/).
 See [the license file](LICENSE.md) for more information. 
 
-## Setup and Build
+## Development
+
+### Setup
 
 In order to build the book, you need to first install Docker 
 (instructions here: [https://docs.docker.com/get-docker/](https://docs.docker.com/get-docker/)).
 
-You can then build the book by running
+### Contributing 
+Primary development in this repository happens on the `dev` branch. If you want to contribute to the book,
+please branch off of `dev` and make a pull request into `dev`.
+
+The `main` branch contains the source material for the live, publicly viewable HTML book.
+
+### Build locally
+
+You can build the book on your own machine by running
 ```
 ./build_html.sh
 ```
 in the root directory of this repository. The book can be viewed in your browser by opening the `source/_build/html/index.html` file.
 
+### Update build environment
+
+You can update the build environment for the book by making changes to `Dockerfile` in the root of the repository in the `main` branch.
+If you push any changes to the `Dockerfile` on the `main` branch, GitHub will trigger a rebuild of the docker image,
+push it to DockerHub, and update the `build_html.sh` script and book deploy GitHub action with the new image digest.
+
+### Update public html
 
+You can update the live, publicly viewable HTML book by making changes to the `source/` folder in the `main` branch.
+If you push any changes to the `source/` folder on the `main` branch, GitHub will trigger a rebuild of the public HTML site.