Create a Security Policy

[joycebrum]

A Security Policy is a GitHub standard document (SECURITY.md) that can be found in the "Security Tab" to instruct users how to report vulnerabilities in a safe and efficient way.

It is a Scorecard Recommendation (being one check of medium priority) and a Github Recommendation.

Together with this issue I'll submit one suggestion of Security Policy, feel free to edit it directly or ask me for editions until it is in compliance with how RxJava would best handle vulnerability reports.