Add contributing.md file

[devendran-m]

Please consider the following when creating a PR:

• Provide a useful description of this PR, suitably verbose, aimed at helping reviewers and contributors

• This PR is created to add CONTRIBUTING.md file to the repository. The content of the CONTRIBUTING.md file contains details to the users/community members on how they can contribute, process they have follow when contributing and steps to adhere to during the process.

• Update all relevant changelogs

• Changelog;

        new file:   CONTRIBUTING.md
        new file:   images/casper_pull_request_flow.png

• Provide a link to the GitHub issue relating to this PR

• N/A

• Identify if any downstream impact as in to, SDKs, SmartContracts etc

• This is a information document for community members, users and therefore no downstream impact is anticipated.

[devendran-m]

Dear @MParlikar , @sacherjj , @ACStoneCL , @ipopescu I have created this PR to add CONTRIBUTING.md file to the casper-node repo as per the direction from Medha. Could you review the content and let me know your comments?

[devendran-m]

Hi @ipopescu ,

Thank you very much for your feedback/comments. I have now incorporated the comments. Please review and let me know your feedback.

[ipopescu]

@devendran-m, there are still unresolved comments. Please take a look and let me know if you need help. Also, please fix the linting.

[devendran-m]

Housekeeping - Moving it to backlog for future sprint planning