docs(faq): show tox config for different use cases

docs/faq.md

@@ -57,20 +57,58 @@ requires = ["poetry-core>=1.0.0"]
 build-backend = "poetry.core.masonry.api"
 ```
 
-And use a `tox.ini` configuration file similar to this:
+`tox` can be configured in multiple ways. It depends on what should be the code under test and which dependencies
+should be installed.
 
-```INI
+#### Usecase #1
+```ini
 [tox]
 isolated_build = true
-envlist = py27, py37
 
 [testenv]
-allowlist_externals = poetry
+deps =
+    pytest
 commands =
-    poetry install -v
-    poetry run pytest tests/
+    pytest tests/ --import-mode importlib
 ```
 
+`tox` will create an `sdist` package of the project and uses `pip` to install it in a fresh environment.
+Thus, dependencies are resolved by `pip`.
+
+#### Usecase #2
+```ini
+[tox]
+isolated_build = true
+
+[testenv]
+whitelist_externals = poetry
+commands_pre =
+    poetry install --no-root --sync
+commands =
+    poetry run pytest tests/ --import-mode importlib
+```
+
+`tox` will create an `sdist` package of the project and uses `pip` to install it in a fresh environment.
+Thus, dependencies are resolved by `pip` in the first place. But afterwards we run Poetry,
+ which will install the locked dependencies into the environment.
+
+#### Usecase #3
+```ini
+[tox]
+isolated_build = true
+
+[testenv]
+skip_install = true
+whitelist_externals = poetry
+commands_pre =
+    poetry install
+commands =
+    poetry run pytest tests/ --import-mode importlib
+```
+
+`tox` will not do any install. Poetry installs all the dependencies and the current package an editable mode.
+Thus, tests are running against the local files and not the builded and installed package.
+
 ### I don't want Poetry to manage my virtual environments. Can I disable it?
 
 While Poetry automatically creates virtual environments to always work isolated