README.md: Add URL of upstream

Author: cclauss

README.md

@@ -1,6 +1,8 @@
 A utility that converts [TAP version 13](https://testanything.org/) to [JUnit](https://junit.org/junit5/). That's it.
 
-The syntax expected is currently pretty custom-tailored for use at https://ci.nodejs.org.
+Upstream is currently unmaintained at https://bitbucket.org/fedoraqa/pytap13/src/develop/
+
+The syntax expected is currently pretty custom-tailored for use at https://ci.nodejs.org
 
 Improvements welcome.