License change

The README currently specifies this as being released under public domain, which can lead to issues in some countries (including the US, where I am). I'm proposing that we release this under MIT instead, as this is the license used by the X11 libraries that are wrapped by this project. Please reply with "yes" if you agree to this change. If I get a "yes" from most contributors but not all, I'll rewrite any remaining contributions myself before pushing this change.

~~~Also, I'm moving this project to [GitLab](https://gitlab.com/daggerbot/x11-rs) unless somebody else takes over as maintainer.~~~

Edit: The main concern is that "public domain" does not provide the protection of a warrantly clause. I don't know of a particular time that this has been a problem, but it's better to be safe.

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