Add PR and issues template

issue_template.md

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+Please note that the Vagrant issue tracker is reserved for bug reports and
+enhancements. For general usage questions, please use the Vagrant mailing list:
+https://groups.google.com/forum/#!forum/vagrant-up. Thank you!
+
+### Vagrant version
+Run `vagrant -v` to show the version. If you are not running the latest version
+of Vagrant, please upgrade before submitting an issue.
+
+### Host operating system
+This is the operating system that you run locally.
+
+### Guest operating system
+This is the operating system you run in the virtual machine.
+
+### Debug output
+Provide a link to a GitHub Gist containing the complete debug output:
+https://www.vagrantup.com/docs/other/debugging.html. Do NOT paste the debug
+output in the issue, just paste the link to the Gist.
+
+### Expected behavior
+What should have happened?
+
+### Actual behavior
+What actually happened?
+
+### Steps to reproduce
+1.
+2.
+3.
+
+### References
+Are there any other GitHub issues (open or closed) that should be linked here?
+For example:
+- GH-1234
+- ...

pull_request_template.md

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+### Overview
+Provide a one-sentence overview of what this pull request is for.
+
+### References
+Please list all issue references. If this fixes an issue, say "Fixes <number>".
+If there is not an existing issue, please open an issue _first_, and then submit
+your pull request.
+- Fixes GH-1234
+- References GH-5678