docs: Add tools/test-lib to README.
It's important to run these tests if you're modifying
the bot library itself.  The tests are really fast,
so there's no harm in regular users running them, so
they get a sanity check that the libraries themselves
are stable.
@@ -62,7 +62,7 @@ To run the tests for
* *zulip*: run `./tools/test-zulip`
-* *zulip_bots*: run `./tools/test-bots`
+* *zulip_bots*: run `./tools/test-lib && ./tools/test-bots`
* *zulip_botserver*: run `./tools/test-botserver`
