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.

Author: cooljeanius

.github/CONTRIBUTING.md

@@ -0,0 +1,18 @@
+## Contributing to GCC
+
+Thanks for taking the time to contribute to GCC! Please be advised that if you are
+viewing this on `github.com`, that the mirror there is unofficial and unmonitored.
+The GCC community does not use `github.com` for their contributions. Instead, we use
+a mailing list (`[email protected]`) for code submissions, code
+reviews, and bug reports.
+
+Perhaps one day it will be possible to use [GitGitGadget](https://gitgitgadget.github.io/) to
+conveniently send Pull Requests commits to GCC's mailing list, the way that the Git project currently allows it to be used to send PRs to their mailing list, but until that day arrives, please send your patches to the mailing list manually.
+
+Please read ["Contributing to GCC"](https://gcc.gnu.org/contribute.html) on the main GCC website
+to learn how the GCC project is managed, and how you can work with it.
+In addition, we highly recommend you to read [our guidelines for read-write Git access](https://gcc.gnu.org/gitwrite.html).
+
+Or, you can follow the ["Contributing to GCC in 10 easy steps"](https://gcc.gnu.org/wiki/GettingStarted#Basics:_Contributing_to_GCC_in_10_easy_steps) section of the ["Getting Started" page](https://gcc.gnu.org/wiki/GettingStarted) on [the wiki](https://gcc.gnu.org/wiki) for another example of the contribution process.
+
+Your friendly GCC community!

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,5 @@
+Thanks for taking the time to contribute to GCC! Please be advised that if you are
+viewing this on `github.com`, that the mirror there is unofficial and unmonitored.
+The GCC community does not use `github.com` for their contributions. Instead, we use
+a mailing list (`[email protected]`) for code submissions, code reviews, and
+bug reports. Please send patches there instead.