fix link to CONTRIBUTING guidelines

closes mrtazz#91

Author: mrtazz

README.md

@@ -237,7 +237,7 @@ merged as fast as possible.
 [libcurl]: http://curl.haxx.se/libcurl/
 [gtest]: http://code.google.com/p/googletest/
 [packagecloud]: https://packagecloud.io/mrtazz/restclient-cpp
-[contributing]: https://github.com/mrtazz/restclient-cpp/blob/master/CONTRIBUTING.md
+[contributing]: https://github.com/mrtazz/restclient-cpp/blob/master/.github/CONTRIBUTING.md
 [curl_keepalive]: http://curl.haxx.se/docs/faq.html#What_about_Keep_Alive_or_persist
 [curl_threadsafety]: http://curl.haxx.se/libcurl/c/threadsafe.html
 [restclient_response]: http://code.mrtazz.com/restclient-cpp/ref/struct_rest_client_1_1_response.html