Document path argument to ruby_test (#80)

It took a while to figure this out... see #79.

Author: sayrer

examples/simple_script/BUILD.bazel

@@ -31,6 +31,8 @@ ruby_test(
     ] + glob([
         "spec/**/*.rb",
     ]),
+    # This is a WORKSPACE-relative path to a spec directory.
+    # It could be longer, like "my/path/to/spec".
     args = [
         "spec",
     ],