Doc: add security policy file for CBDB (#110)

Add new SECURITY.md file for cloudberry Database, which shows that
community how to report security issues, and rules on the security
issues. This version is 1.0. We need to update the policy along with our
community grows.

SECURITY.md

@@ -0,0 +1,49 @@
+Thanks for helping make Cloudberry Database safe!
+
+---
+
+## Reporting Security Issues
+
+To report a security issue, please email
+[security@cloudberrydb.org](mailto:security@cloudberrydb.org). This
+project follows a 90-day disclosure timeline. We will publish the
+[security
+advisories](https://github.com/cloudberrydb/cloudberrydb/security/advisories)
+via GitHub.
+
+You should receive a response within 2 weeks. If for some reason you
+do not, please follow up via email to ensure we received your original
+message.
+
+Please include the requested information listed below (as much as you
+can provide) to help us better understand the nature and scope of the
+possible issue:
+
+* Type of issue (e.g. buffer overflow, SQL injection, cross-site
+  scripting, etc.)
+* Full paths of source file(s) related to the manifestation of the
+  issue
+* The location of the affected source code (tag/branch/commit or
+  direct URL)
+* Any special configuration required to reproduce the issue
+* Step-by-step instructions to reproduce the issue
+* Proof-of-concept or exploit code (if possible)
+* Impact of the issue, including how an attacker might exploit the
+  issue
+
+This information will help us triage your report more quickly.
+
+## Do not
+
+For better collaboration, we hope you:
+
+- Do not file public issues on GitHub for security vulnerabilities.
+- Do not report non-security-impacting bugs through this channel. If
+  you have any questions on using, development, please use [GitHub
+  Issues, Discussions or
+  Slack](https://github.com/cloudberrydb/cloudberrydb/issues/new/choose)
+  instead.
+
+## Preferred Languages
+
+We prefer all communications to be in English.