docs: add tests badge

Author: kelly-sovacool

README.md

@@ -2,5 +2,7 @@
 
 Code Ocean capsule: MOSuite - filter low counts
 
+[![tests](https://github.com/CCBR/MOSuite-filter-counts/actions/workflows/tests.yml/badge.svg)](https://github.com/CCBR/MOSuite-filter-counts/actions/workflows/tests.yml)
+
 - [Code Ocean Capsule](https://poc-nci.codeocean.io/capsule/2922767/tree)
 - [MOSuite R package docs](https://ccbr.github.io/MOSuite/)