Add files to discourage submissions of PRs to the GitHub mirror.

Currently there is an unofficial mirror of GCC on GitHub that people
sometimes submit pull requests to:
https://github.com/gcc-mirror/gcc
However, this is not the proper way to contribute to GCC, so that means
that someone (usually Jonathan Wakely) has to go through the PRs and
manually tell people that they're sending their PRs to the wrong place.
One thing that would help mitigate this problem would be files in a
special .github directory that GitHub would automatically open when
contributors attempt to open a PR, that would then tell them the proper
way to contribute instead. This patch attempts to add two such files.
They are written in Markdown, which I'm realizing might require some
special handling in this repository, since the ".md" extension is also
used for GCC's "Machine Description" files here, but I'm not quite sure
how to go about handling that. Also note that I adapted these files from
equivalent files in the git repository for Git itself:
https://github.com/git/git/blob/master/.github/CONTRIBUTING.md
https://github.com/git/git/blob/master/.github/PULL_REQUEST_TEMPLATE.md
What do people think?

ChangeLog:

	* .github/CONTRIBUTING.md: New file.
	* .github/PULL_REQUEST_TEMPLATE.md: New file.

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