security: add SECURITY.md and issue template

Author: ghuntley

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,4 @@
+contact_links:
+  - name: Security
+    url: https://www.gitpod.io/security
+    about: Please report security vulnerabilities here.

SECURITY.md

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+# Reporting Security Issues
+
+We appreciate your efforts to responsibly disclose your findings, and will make every effort to acknowledge your contributions.
+
+To report a security issue please visit https://gitpod.io/security