Add link to self-test instructions.

Author: mikeschinkel

README.md

@@ -7,3 +7,7 @@
 We are in the middle of updating our documentation.  
 
 If you are interested in our progress you can [find it here](https://wplib.github.io/box-docs/).
+
+## WANT TO CONTRIBUTE?
+
+Help us [**write tests**](https://github.com/wplib/wplib-box/issues/538) for WPLib Box. _(It is actually really easy, assuming you know Bash.)