Update Contributing.md

[arjunsinghy96]

Update the contributing.md with basic contribution guidelines. Few things can be:

• all code should be tested
• don't use the master branch
• Make atomic commit such that they solve one and only one particular issue.
• The commit should refer to the issue they solve.
• rebase branch to current master before opening a PR

[arjunsinghy96]

Should contain

• Commit guideline. Summarize coala good commit

• Rebase the branch to master before creating the PR (Idea is to maintain linear commit history)

• Request to get assigned to the issue before taking it up.

• Don't try to work on already assigned issue.

and similar things. More suggestion are welcome

[arjunsinghy96]

Request to get assigned to this issue. First to request gets assigned.