docs: update readme-heroku.md to match the current reality

readme-heroku.md

@@ -1,16 +1,26 @@
 Our production Heroku deployment is controlled by the following files:
  - `Procfile`: tells Heroku what command to launch in each Dyno;
  - `runtime.txt`: tells Heroku which run-time engine to use (i.e., which version of Python);
+ - `requirements.txt`: tells Heroku what our production dependencies are;
+ - `bin/post_compile`: Heroku builds run this after doing `pip install -r requirements.txt`.
 
-   Heroku detects Python by default, but `runtime.txt` lets us specify/bump the version as needed;
- - `requirements.txt`: tells Heroku what our production dependencies are.
+Updating dependencies:
+ - Our dependencies are declared in `pyproject.toml`, edit this file to modify them.
+ - `requirements.txt` is a generated "lock" file. To update it, run:
+
+       hatch env remove prod
+       rm requirements.txt
+       hatch env create prod
+
+   It is also possible to edit it manually, e.g., to fix a critical vulnerability,
+   but an occasional full rebuild is a good idea to stay up to date.
 
 Updating `g2p`:
  - By default, `g2p` only gets updated for `readalong-studio` on Heroku when:
-   - we make a new release of `g2p` on PyPI
-   - we update `requirements.min.txt` here to ask for that release
+   - we make a new release of `g2p` on PyPI, **and**
+   - we update `requirements.txt` here to ask for that release
  - Manual override: it is also possible to update g2p to the current commit on the `main` branch using this procedure:
-   - Change `requirements.min.txt` to specify `g2p @ git+https://github.com/roedoejet/g2p@main`.
+   - Change `requirements.txt` to specify `g2p @ git+https://github.com/roedoejet/g2p@main`.
    - Commit that change to `main` in a sandbox connected to Heroku.
    - `git push heroku main`
    - This will force a build which will fetch the main branch of g2p from GitHub.