docs: add a code of conduct

As previously discussed in today's meeting, this commit adds a code
of conduct to winit. I've elected to re-use the Rust project's Code
of Conduct, as it is also in use by other popular Rust projects and I
agree with its intentions. The goal is to set forward a set of explicit
expectations for how discourse in the rust-windowing project should be
conducted.

I've deliberately left out a mechanism for enforcing this code of
conduct. My hope is that, by the act of setting these expectations,
contributors will voluntarily follow them. In addition I have no
interest in being an internet janitor. If this becomes necessary in the
future we can look into it later.

rust-windowing collaborators, please read the linked Code of Conduct.
Not only for expectations of behavior, but also for expectations of how
it will be enforced should it come to it.

Signed-off-by: John Nunley <dev@notgull.net>

CODE_OF_CONDUCT.md

@@ -0,0 +1,6 @@
+# Code of Conduct
+
+The `rust-windowing` project adheres to the [Rust Code of Conduct]. This
+describes the minimum behavior expected from all contributors.
+
+[Rust Code of Conduct]: https://www.rust-lang.org/policies/code-of-conduct