Update support badge (#171)

Update README.md

README.md

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 # Composer-enabled WordPress template
 
-[![Actively Maintained](https://img.shields.io/badge/Pantheon-Actively_Maintained-yellow?logo=pantheon&color=FFDC28)](https://pantheon.io/docs/oss-support-levels#actively-maintained-support)
+[![Actively Maintained](https://img.shields.io/badge/Pantheon-Minimal_Support-yellow?logo=pantheon&color=FFDC28)](https://pantheon.io/docs/oss-support-levels#minimal-support)
 [![GitHub Release](https://img.shields.io/github/v/release/pantheon-systems/wordpress-composer-managed)](https://github.com/pantheon-upstreams/wordpress-composer-managed)
 
 This is Pantheon's recommended starting point for forking new [WordPress](https://wordpress.org) upstreams that work with the Platform's Integrated Composer build process.