Improve documentation for upgrade scenario

If one upgrade from previous molecule versions, and since drivers are now
provided by the molecule-plugins, molecule-podman|docker|vagrant
packages needs to be removed.

See:
- ansible#3883
- ansible#3895

docs/installation.md

@@ -95,6 +95,13 @@ installation command would look like this:
 $ python3 -m pip install --user "molecule-plugins[podman]"
 ```
 
+!!! warning
+
+    If you upgrade molecule from previous versions, make sure to remove
+    previously installed drivers like for instance `molecule-podman` or
+    `molecule-vagrant` since those are now available in the `molecule-plugins`
+    package.
+
 Installing molecule package also installed its main script `molecule`,
 usually in `PATH`. Users should know that molecule can also be called as
 a python module, using `python -m molecule ...`. This alternative method