Add pointer to github marketplace

Author: ianfixes

README.md

@@ -3,6 +3,7 @@
 [![Gem Version](https://badge.fury.io/rb/arduino_ci.svg)](https://rubygems.org/gems/arduino_ci)
 [![Documentation](http://img.shields.io/badge/docs-rdoc.info-blue.svg)](http://www.rubydoc.info/gems/arduino_ci/1.1.0)
 [![Gitter](https://badges.gitter.im/Arduino-CI/arduino_ci.svg)](https://gitter.im/Arduino-CI/arduino_ci?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
+[![GitHub Marketplace](https://img.shields.io/badge/Get_it-on_Marketplace-informational.svg)](https://github.com/marketplace/actions/arduino_ci)
 
 You want to run tests on your Arduino library (bonus: without hardware present), but the IDE doesn't support that.  Arduino CI provides that ability.
 
@@ -12,6 +13,8 @@ You want your Arduino library to be automatically built and tested every time so
 
 `arduino_ci` is a cross-platform build/test system, consisting of a Ruby gem and a series of C++ mocks.  It enables tests to be run both locally and as part of a CI service like GitHub Actions, TravisCI, Appveyor, etc.  Any OS that can run the Arduino IDE can run `arduino_ci`.
 
+> Note: for testing GitHub pull requests, an [Arduino CI GitHub Action](https://github.com/marketplace/actions/arduino_ci) is available for your convenience.  This method of running `arduino_ci` is driven by Docker, which may also serve your local testing needs (as it does not require a ruby environment to be installed)
+
 
 Platform | CI Status
 ---------|:---------