doc: Update SECURITY.md to include Github's reporting mechanism

.github/SECURITY.md

@@ -5,7 +5,10 @@
 
 The Gitblit team takes security bugs seriously. We appreciate your efforts to responsibly disclose your findings, and will make every effort to acknowledge your contributions.
 
-To report a security issue, please send an email to the following email address and include the word "SECURITY" in the subject line.
+
+To report a security vulnerability, you can use the Github mechanism to [privately report a vulnerability](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing-information-about-vulnerabilities/privately-reporting-a-security-vulnerability). On Gitblit's repository page, choose the `Security` tab (under the repository name). Click the `Report a vulnerability` button on the right. 
+
+Alternatively, you can also report any security issue via e-mail. Send an email to the following email address and include the word "SECURITY" in the subject line.
 
 ```
 gitblitorg@gmail.com