Add readme for testchecks directory.

checkserver/testchecks/README.md

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+Example checks for testing
+---------
+
+These checks are short, don't do much, and are here for testing out the check server. To run them, try this:
+
+    python server.py -d testchecks/
+
+And go to one of the following URLs:
+
+    http://localhost:8111/check/slow?arg=hello
+    http://localhost:8111/check/fast?arg=world
+
+The fast check is fast, the slow check takes five seconds.