Create SECURITY.md (#452)

* Create SECURITY.md

* Update supported versions section

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+pg_stat_monitor project follows rolling release strategy. So all security updates go to new versions.
+
+## Reporting a Vulnerability
+
+Please report any vulnerabilities to our project in [Jira](https://perconadev.atlassian.net/jira/software/c/projects/PG/issues).
+
+If the vulnerability is accepted and confirmed by our experts, you should normally expect us to deliver
+a version with a fix according to the timelines provided below:
+
+For Percona created software (our engineers wrote the code):
+
+- Low/Medium: 120 days
+- High: 90 days
+- Critical: ASAP but should not exceed 30 days
+
+For Non-Percona created software (upstream provided/packaged) from the time the vendor releases a patch:
+
+- Low/Medium: 2nd release from current version
+- High: Next release
+- Critical: Hotfix or no later than next release (our regular release cadence is once every month)