Mention pypi / pipx as simple installation option

Signed-off-by: David Linke <david.linke@catalysis.de>

README.md

@@ -48,6 +48,11 @@ filter-repo requires:
 installation for basic use on many systems trivial: just place that
 file into your $PATH.
 
+`git-filter-repo` is also available as 
+[PyPI-package](https://pypi.org/project/git-filter-repo/). Therefore, it
+can be [pipx](https://pypa.github.io/pipx/)-installed (also on Windows): 
+`pipx install git-filter-repo`
+
 See [INSTALL.md](INSTALL.md) for things beyond basic usage or special
 cases.  The more involved instructions are only needed if one of the
 following apply: