docs: Add security issues section to README (#157)

* chore: Add security issues section to README

* Update README.rst

---------

Co-authored-by: Ned Batchelder <ned@edx.org>

Author: adzuci

README.rst

@@ -203,3 +203,8 @@ run the code, and returns the output and exit status of the process.
 Python code, and runs it using jail_code, modifying the globals dictionary as a
 side-effect.  safe_exec does this by serializing the globals into and out of
 the subprocess as JSON.
+
+Reporting Security Issues
+-------------------------
+
+Please do not report security issues in public. Please email security@openedx.org.