[AC-649] Add Contribution Guidelines to README

[ribhavsharma]

Description of what I changed

Added the Contribution Guidelines to README.md

Issue I worked on

JIRA Issue: https://issues.openmrs.org/browse/AC-649

Checklist: I completed these to help reviewers :)

• My pull request only contains ONE single commit
  (the number above, next to the 'Commits' tab is 1).

• My pull request is based on the latest changes of the master branch.

[codecov-io]

Codecov Report

Merging #671 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #671   +/-   ##
======================================
  Coverage    12.6%   12.6%           
======================================
  Files         232     232           
  Lines       10630   10630           
  Branches      990     990           
======================================
  Hits         1340    1340           
  Misses       9209    9209           
  Partials       81      81

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