add greetings workflow (to staging)

[saum7800]

Through this PR, I am creating a GitHub action to reply with an automated message to first time contributors. When a user makes a PR for the first time or it is their first time creating an issue, then an automated response is given to the contributor. Following is a screenshot of an automated response for a first time PR. Since I cannot create issues on Forks, I am unable to test the workflow for issues but it shall run in the same manner as PRs. This PR will resolve #97 .