Add issues/PR templates

[nathanlct]

Thank you for contributing to Nocturne!

Please try to keep pull requests a reasonable length, make sure tests are passing and that your new code is well commented and follows our coding style.

Description

PR/Issues templates. I believe we can't see them on Github if they're not on the main branch.

Changes

• Add a simple pull request template
• Add issue form templates (bug report, feature request, question) - leaving the option to create a blank issue in case templates are not exhaustive, which we can disable later if we seem fit
• Add a "contact us privately" option when creating an issue (@eugenevinitsky linked this to your email since you're the paper's corresponding author - we can change it if you don't want that)
• Add a "feel free to open an issue" paragraph in the README

Related

Fixes #23

[eugenevinitsky]

Thank you for doing this!