[Documentation] update the CONTRIBUTING.md with Issues & Pull Requests sections

[sandramsc]

Link to issue: getnighthawk

[welcome]

Thanks for opening this issue. A contributor will be by to give feedback soon. In the meantime, please review the Layer5 Community Welcome Guide and sure to join the community Slack.

[sandramsc]

Hi, (only if there's a need to do so in this case) can I have this issue assigned to me, please?

• currently working on this issue in the Layer5 repo, creating a new issue in each repo mentioned so as to effectively maintain the procedure of creating an issue and thereafter a PR that closes it.