Add guidelines for lightning.qubit repository

[mlxd]

Context: Currently there are no repository guidelines for creating issues, PRs, or general contribution information. This PR will add support for this.

Description of the Change: Added CODE_OF_CONDUCT.md, CONTRIBUTING.md ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md modeled on main PennyLane repository.

Benefits: Enables users to contribute more easily, and adds guidelines for creating PRs.

Possible Drawbacks: None

Related GitHub Issues:

[codecov]

Codecov Report

Merging #109 (d00ba53) into master (ec87485) will not change coverage.
The diff coverage is n/a.

❗ Current head d00ba53 differs from pull request most recent head 353b0db. Consider uploading reports for the commit 353b0db to get more accurate results

@@           Coverage Diff           @@
##           master     #109   +/-   ##
=======================================
  Coverage   98.14%   98.14%           
=======================================
  Files           3        3           
  Lines          54       54           
=======================================
  Hits           53       53           
  Misses          1        1           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ec87485...353b0db. Read the comment docs.

[trbromley]

Thanks @mlxd, looks great!

[trbromley]

Nice, I didn't know you could do links like this! But it doesn't work, does it need a ../tests/? This is the URL it sends to:
https://github.com/PennyLaneAI/pennylane-lightning/blob/repo_guidelines/.github/tests

[mlxd]

Good catch! I forgot to add a sufficient number of ../. I'll update this to be absolute to the tests dir, rather than relative.

[mlxd]

On second thought, the relative path you suggested is much tidier.