Add section on security fixes, backports, backwards-incompatible fixes

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: sethmlarson

security/policy.rst

@@ -144,6 +144,32 @@ Here's what to expect for how a vulnerability report will be handled:
 For more information about how the PSRT handles vulnerabilities,
 see :ref:`psrt-vulnerability-process`.
 
+When are security fixes released or backported?
+-----------------------------------------------
+
+Advisories and CVE records are published after a security fix is first merged.
+The CVE record contains information about the patch and will be kept up-to-date
+as the patch is backported and released to Python versions.
+
+According to :pep:`602`, Python versions receive five years of security updates
+after the new Python version is released. However, these security updates
+are not published according to a fixed schedule or as a result of any security
+fix being available. The schedule for security updates is determined by
+the Release Manager. Users are encouraged to apply patches themselves ahead
+of patches being merged and released if necessary.
+
+Security fixes that only affect binary installers (such as macOS and Windows)
+will not be published to Python release streams that are
+:ref:`source-only <secbranch>`.
+
+Some security fixes may require introducing backwards-incompatible behavior
+changes. In these circumstances, the Python Steering Council
+will decide with the relevant Release Managers whether to introduce
+a backwards incompatible change to fix a security vulnerability
+in a stable version. This may affect whether a security fix is backported
+to all Python versions or enabled by default. See the CVE record for
+more information.
+
 Code of conduct
 ---------------