github: try to get contributors to read contribute.md

How hard can it be? I know contribute.md is a shitty wall of text, but
that doesn't make it less important, and each violation will make it
take longer by adding another review round anyway.

And we really don't need lazy pull requests. If you can't be assed to
follow a few simple rules, your code is probably shit or you wanted to
be quick and lazy. Why should we accept it? We're the ones who have to
maintain it and fix bugs in it, and if the contributor is lazy, the
chance of you maintaining it is probably slim as well. On the other
hand, WE the maintainers are not obligated to anything.

Don't say that though, the first contact doesn't need to be negative. I
don't know if the "lazy pull requests" is still too strong, but I can't
tell.

.github/PULL_REQUEST_TEMPLATE

@@ -1,6 +1,5 @@
-Ideally, you shouldn't need enter any text here, and your commit messages should
+Read this before you submit this pull request:
-explain your changes sufficiently (especially why they are needed). Read
+https://github.com/mpv-player/mpv/blob/master/DOCS/contribute.md
-https://github.com/mpv-player/mpv/blob/master/DOCS/contribute.md for coding
-style and development conventions.
 
-Remove this text block.
+Reading this link and following the rules will get your pull request reviewed
+and merged faster. Nobody wants lazy pull requests.