[Chore] Add a contributing.md guide + Templates for Bug Reports / Feature Requests

[avishj]

It would be really helpful to have these. I am happy to take these on and to work on the PDF feature I opened in #177. Just want to confirm that it is okay with the maintainers.

[avishj]

Relevant links:

1. https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors
2. https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository