Update contributing guidelines.

Author: nicoulaj

README.md

@@ -64,11 +64,13 @@ Contributing
 
 Contributions are welcome, just make sure you follow the guidelines:
 
+ * Completions are not accepted when already available in zsh.
  * Completions are not accepted when already available in their original project.
  * Please do not just copy/paste someone else completion, ask before.
  * Completions only partially implemented are not accepted.
  * Please add a header containing authors, license info, status and origin of the script (example [here](src/_ack)).
  * Please try to follow [Zsh completion style guide](https://github.com/zsh-users/zsh/blob/master/Etc/completion-style-guide).
+ * Please send one separate pull request per file.
  * Send a pull request or ask for committer access.