Security: Publish security policy and reporting guidelines

SECURITY.md

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+# Security
+
+Prebid.org takes the security of our software seriously. If you believe you have found a security vulnerability in software managed in a Prebid.org-managed repository, please report it to us as described below.
+
+## Reporting Security Issues
+
+**Please do not report security vulnerabilities through public GitHub issues or PRs.**
+
+Instead, please report them via private reporting (e.g. https://github.com/prebid/prebid-server-java/security) or via email at security (at) prebid.org.
+
+You should receive a response within 3 business days. If for some reason you do not, please follow up via email to ensure we received your original message.
+
+Please include as much of the requested information below as possible 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.