In the answers of What's the canonical retort to "it's open source, submit a patch"?, many people voiced the opinion that simply asking people to submit a patch is arrogant and rude.
But it seems to me that as a developer on any open source project, you will see many more feature requests on the mailing list than you could possibly implement. So when a user says, "I would like to see feature X", the truth of the matter is usually that the chances of it getting implemented are pretty slim unless they submit a patch themselves. Also, sometimes a little encouragement might be all that's needed to turn a user into a contributor.
On the other hand, you don't want to scare (potential) contributors away by coming off as rude.
So how would you say "please submit patches instead of asking for features" in a friendly manner?
Update: Thanks for all the suggestions! I see most of them require pretty lengthy explanations. But since I'd rather avoid either (a) explaining the same thing every other day (it just takes too much time), or (b) using snippets that I paste into email (it gets impersonal real quick), I wonder: Has anyone written this up in a document that I can link to?
(Project-specific things like how to write tests, compile the code, and submit the patch still need to be documented of course, but I think those technical issues should go into CONTRIBUTING.txt anyway.)