Add link to python-appimage

Author: niess

README.md

@@ -1,3 +1,6 @@
+**Manylinux Python AppImages are now available from the new [python-appimage](https://github.com/niess/python-appimage) project. Building basic Python apps has also been made easier with the latter.**
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+---
 
 # A Python plugin for linuxdeploy [![Build Status](https://travis-ci.com/niess/linuxdeploy-plugin-python.svg?branch=master)](https://travis-ci.com/niess/linuxdeploy-plugin-python)