Add new Contribution guidelines

[jaladh-singhal]

This contribution guidelines page is specifically written in informal way to get a new contributor motivated. Besides, it's purposedly kept short-average length (pointing to other guidelines where required) to not to overwhelm new contributor. A good checklist to cross-check what I have written here can be tips in this article - let me know what did I miss and any other improvements.

Demo of how this looks after rendering: https://github.com/jaladh-singhal/tardis/blob/contrib-guide/CONTRIBUTING.md

To-dos

• Create issue for creating doc pages/guidelines for void links i.e marked by # only
• Create issue for updating running tests guidelines (i.e. outdated) which is used here
• Put the removed commit message guidelines at somewhere else, let me know where?

[codecov]

Codecov Report

Merging #1237 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1237   +/-   ##
=======================================
  Coverage   80.63%   80.63%           
=======================================
  Files          41       41           
  Lines        3424     3424           
=======================================
  Hits         2761     2761           
  Misses        663      663           

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