Update contributing.md

CONTRIBUTING.md

@@ -5,6 +5,7 @@ All contributions are welcome, but please take a note of the following before su
 - Make sure your changes are properly tested. It's easy to break stuff, especially new card implementations, so please test it properly and look for any edge cases that could cause bugs with other cards in play at the same time.
 - We release cards in order by their set releases. Before work is done on any cards from the next set we should first finish up implementations for all the cards in the current set and fix bugs before starting on a new one.
 - If you're a new contributor working on new cards, please submit one pull request per card implementation. This is to avoid cases where a pull request becomes stale because the maintainer spots an issue with one of the cards, blocking all the other ones from being merged too.<br><br>For those comfortable with the codebase and with previous contributions, feel free to submit smaller batches of cards in your pull requests. You know who you are.
+- Please include a description in your PR explaining what it does and why
 
 ---