Improve cache Poetry install instructions

See #150.

README.md

@@ -509,6 +509,51 @@ jobs:
 
 The directory to cache will depend on the operating system of the runner.
 
+Note that when the cache hits and the *Install Poetry* step is skipped, all configurations done during install are skipped. Therefore, the loaded Poetry will run with default settings.
+
+This may break the cache of Python dependencies when the `.lock` file changes and the Poetry *install* is recovered from an old cache. For example, if you set
+
+```yaml
+with:
+  virtualenvs-in-project: true
+```
+
+and have a cache step for `.venv`, the `install-poetry` will run and configure Poetry to create the venv in `.venv` *the first time it is run*. Dependencies will install and will be cached. However, when you update your `.lock` file, the Poetry installation will be recovered from cache, and a new venv will be created in the default Poetry location (not `.venv`, because the install poetry step doesn't cache the settings), so the cache venv step will fail (as there's no `.venv` directory to cache).
+
+To remedy this, you should use this instead
+
+```yaml
+name: test
+
+on: pull_request
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+      - name: Set up python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+      - name: Load cached Poetry installation
+        id: cached-poetry
+        uses: actions/cache@v3
+        with:
+          path: ~/.local  # the path depends on the OS
+          key: poetry-0  # increment to reset cache
+      - name: Install Poetry
+        if: steps.cached-poetry.outputs.cache-hit != 'true'
+        uses: snok/install-poetry@v1
+        with:
+          virtualenvs-create: true
+          virtualenvs-in-project: true
+      - name: Configure poetry
+        if: steps.cached-poetry.outputs.cache-hit == 'true'
+        run: poetry config virtualenvs.in-project true
+```
+
 ## Contributing
 
 Contributions are always welcome; submit a PR!