Update FAQ to better explain running FD outside of project env

The investigations in issue #460 reveal that for now we are likely less
backwards-compatible than we'd like to be. Until that situation is
improved, let's tell users to open an issue if they encounter problems.
This should help us gauge how to prioritize this.

README.md

@@ -574,14 +574,18 @@ run for each `libX`:
 fawltydeps libX
 ```
 
-### Why must FawltyDeps run in the same Python environment as my project dependencies?
+### Does FawltyDeps need to run in the same Python environment as my project?
 
-(This is no longer true since FawltyDeps v0.11: FawltyDeps should be able to
-automatically find your project dependencies when they are installed in a Python
-environment that exists within your project. If your project dependencies are
-installed elsewhere, you can point FawltyDeps in their direction with `--pyenv`,
-as explained above in the section on
-[Python environment mapping](#python-environment-mapping))
+No (not since FawltyDeps v0.11). FawltyDeps should be able to automatically find
+your project dependencies when they are installed in a Python environment that
+exists within your project. If your project dependencies are installed
+elsewhere, you can point FawltyDeps in their direction with `--pyenv`, as
+explained above in the section on
+[Python environment mapping](#python-environment-mapping)).
+
+See also the next question for more details.
+
+### Why does FawltyDeps need a Python environment with my project dependencies?
 
 The reason why FawltyDeps need to find your project dependencies _somewhere_ is
 that the core logic of FawltyDeps needs to match `import` statements in your
@@ -610,14 +614,39 @@ they provide. This is:
   i.e. the one in which FawltyDeps itself is running.
 
 As a final resort, when an installed package is not found for a declared
-dependency, the
-_identity mapping_ that FawltyDeps falls back to will still do a good job for
-the majority of dependencies where the import name is indeed identical to the
-package name that you depend on.
-
-This is an area of active development in FawltyDeps, and we are
-[working on better solutions](https://github.com/tweag/FawltyDeps/issues/195),
-to avoid having to fall back to this identity mapping.
+dependency, the _identity mapping_ that FawltyDeps falls back to will still do
+a good job for the majority of dependencies where the import name is indeed
+identical to the package name that you depend on.
+
+### My project is using Python version before v3.7, can I still use FawltyDeps?
+
+Yes! Even though FawltyDeps itself runs on Python >=v3.7, we try to support
+analyzing projects that run on any version of Python 3.
+
+As explained in the previous two questions, FawltyDeps itself does not need to
+run inside the same Python environment as your project and its dependencies.
+
+You can instead install FawltyDeps using a newer Python version (e.g. via
+[uvx](https://docs.astral.sh/uv/guides/tools/#running-tools) or
+[pipx](https://github.com/pypa/pipx)). Then run FawltyDeps from inside your
+project directory. If your project has an embedded Python environment (e.g.
+under `.venv/`) then FawltyDeps should automatically find it and use it to
+analyze your project dependencies. Alternatively, you can always use `--pyenv`
+to point FawltyDeps to where your dependencies are installed.
+
+Currently the lowest Python version that your project can use (and still be
+analyzed by FawltyDeps) is determined by our use of the
+[`ast` module](https://docs.python.org/3/library/ast.html#module-ast) in the
+Python standard library: As long as your project's Python syntax is compatible
+with the Python version that FawltyDeps runs on, you should be fine. If you run
+into problems with older Python syntax (e.g. using `async` or `await` as
+variable names), please open an issue, and we'll look into extending our
+support further.
+
+A final resort can be to downgrade to an older version of FawltyDeps that is
+compatible with the Python version used in your project. Currently, the only
+Python version we have dropped support for is v3.7, and FawltyDeps v0.18 is
+the last release to support this Python version.
 
 ### Why does FawltyDeps fail to match `sklearn` with `scikit-learn`?