"Your contribution does not fit our current guidelines. Please read CONTRIBUTING.md and feel free to ask if you have any further questions."
comes across very differently than something aggressive.
You're telling the contributor that his/her work does not conform to the project's standards without being an asshole.
I don't see where the difficulty of treating other human beings with some respect resides.
"Your contribution is inadequate. Try again." That's blunt. For one, it's purely negative. It doesn't say what to do instead, only that something wrong was done. It makes people feel like there's no hope for them to do it right.
Let's try it:
"I, speaking not only for myself, would be most grateful if, at least for a period of some number of years during which your already considerable skills continue to attain even greater heights of excellence, you might, as kindly as you have done here, bestow the favor of your well-considered patches upon some other open source software project. Should you be so obliging as to accommodate our wishes in this matter, you shall be rewarded with no less than a half pound sack of jelly babies, delivered to your place of residence by overnight courier."
Now that I look at it, though, not bad! Would it kill those kernel developers to use this friendly template?
Hostility is not avoidable in human interactions; the British have off-the-shelf reusable solutions that satisfy everyone involved with reasonable compromise.