document urchin -sh

readme.md

@@ -138,6 +138,14 @@ To test with multiple shells in sequence, use something like:
       urchin -s $shell ./tests
     done
 
+#### (c) Cross shell tests with `urchin -sh` (experimental)
+If you run urchin with the `-sh` flag, it will be as if you ran
+`$TEST_SHELL`. Unless `$TEST_SHELL` isn't set, in which case it'll
+be as if you ran `/bin/sh`. Putting this in she shebang line might
+eventually work out to be a cleaner way of doing cross-shell testing.
+
+    #!/usr/bin/env urchin -sh
+    test a = a
 
 ## Alternatives to Urchin
 Alternatives to Urchin are discussed in