tests-clay: add a readme file

Signed-off-by: schu <schu-github@schulog.org>

tests-clay/README.md

+Writing Clay tests for libgit2
+==============================
+
+For information on the Clay testing framework and a detailed introduction
+please visit:
+
+https://github.com/tanoku/clay
+
+
+* Write your modules and tests. Use good, meaningful names.
+
+* Mix the tests:
+
+        ./clay
+
+* Make sure you actually build the tests by setting:
+
+        BUILD_CLAY=ON
+
+* Test:
+
+        ./build/libgit2_clay
+
+* Make sure everything is fine.
+
+* Send your pull request. That's it.