Add a repostatus badge to the README

This is set to 'active' and is described here
<https://www.repostatus.org/>

This is now a requirement of F5/NGINX OSS projects.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Author: ac000

README.md

@@ -1,5 +1,7 @@
 # NGINX Unit
 
+[![Project Status: Active – The project has reached a stable, usable state and is being actively developed.](https://www.repostatus.org/badges/latest/active.svg)](https://www.repostatus.org/#active)
+
 ## Universal Web App Server
 
 ![NGINX Unit Logo](docs/unitlogo.svg)