docs: update readme

Add a description for how to run a particular test locally

Author: aschmahmann

README.md

@@ -24,6 +24,12 @@ $ npm install -g ipfs-interop
 $ ipfs-interop
 ```
 
+### Run a particular test locally
+
+```console
+$ node bin/ipfs-interop.js -- -t node --grep {substring-test-name}
+```
+
 ## Testing with different versions of go/js IPFS
 
 ### As a project