Add SECURITY.md [skip ci]

Author: amodm

SECURITY.md

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+# Security Policy
+
+If you're looking to report a security vulnerability, please go to the
+[Security -> Advisories](https://github.com/amodm/webbrowser-rs/security/advisories) section, and click on
+the `Report a vulnerability` button
+
+What to expect:
+* 48hr turnaround to acknowledge the issue
+* The closure ETA can be varied depending on the degree of severity and complexity of resolution
+* I have nothing to offer as a reward, except a virtual hug :hugs: as thanks for your selfless effort, and
+acknowledging your contribution on the release notes.
+