Add ordering rule

CONTRIBUTING.md

@@ -27,6 +27,8 @@ Currently, the only accepted inputs for this field are as follows:
 * `X-Mashape-Key` - _the name of the header which may need to be sent
 * No - _the API requires no authentication to run_
 
+Please continue to follow the alphabetical ordering that is in place.
+
 ##Pull Request
 
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