readme: document the build process more

README.md

@@ -30,11 +30,16 @@ you'll need a browser which supports webGL (e.g. chromium).
 
 ## Building
 
-- `./main.py --export-stl model.stl`
+- `make all` will produce STLs, gcode, etc in the `build/` directory
+- the models can be parameterized with environment variables:
+  - `CASE_RENDER_PHONE=1`  => render not just the case, but also what it would look like with the phone inside
+  - `CASE_RENDER_PHONE_ONLY=1`  => render ONLY the phone, no case
+  - `CASE_BATTERY=<name>`  => design the case to fit a specific battery model, or "none".
+
 
 ## Development
 - `./main.py --editor` or `cq-editor ./main.py` to load an interactive GUI
   - press the green play button to render the model
-  - call with `--render-phone` to see how the phone would fit inside the case
+  - call with `--render-phone` to see how the phone would fit inside the case, or apply any of the environment variables from above.
 
 - CadQuery docs may be found here: <https://cadquery.readthedocs.io/en/latest/quickstart.html>