Add PyPI badge in README

Author: flashcode

README.adoc

@@ -3,6 +3,7 @@
 :email: [email protected]
 :lang: en
 
+image:https://img.shields.io/pypi/v/msgcheck.svg["PyPI", link="https://pypi.org/project/msgcheck/"]
 image:https://github.com/flashcode/msgcheck/workflows/Tests/badge.svg["Tests", link="https://github.com/flashcode/msgcheck/actions"]
 
 `msgcheck` performs various checks on gettext files (with extension ".po"):