SECURITY: describe how to report vulnerabilities

In the same document, describe that Git does not have Long Term Support
(LTS) release trains, although security fixes are always applied to a
few of the most recent release trains.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

SECURITY.md

@@ -0,0 +1,51 @@
+# Security Policy
+
+## Reporting a vulnerability
+
+Please send a detailed mail to git-security@googlegroups.com to
+report vulnerabilities in Git.
+
+Even when unsure whether the bug in question is an exploitable
+vulnerability, it is recommended to send the report to
+git-security@googlegroups.com (and obviously not to discuss the
+issue anywhere else).
+
+Vulnerabilities are expected to be discussed _only_ on that
+list, and not in public, until the official announcement on the
+Git mailing list on the release date.
+
+Examples for details to include:
+
+- Ideally a short description (or a script) to demonstrate an
+  exploit.
+- The affected platforms and scenarios (the vulnerability might
+  only affect setups with case-sensitive file systems, for
+  example).
+- The name and affiliation of the security researchers who are
+  involved in the discovery, if any.
+- Whether the vulnerability has already been disclosed.
+- How long an embargo would be required to be safe.
+
+## Supported Versions
+
+There are no official "Long Term Support" versions in Git.
+Instead, the maintenance track (i.e. the versions based on the
+most recently published feature release, also known as ".0"
+version) sees occasional updates with bug fixes.
+
+Fixes to vulnerabilities are made for the maintenance track for
+the latest feature release and merged up to the in-development
+branches. The Git project makes no formal guarantee for any
+older maintenance tracks to receive updates. In practice,
+though, critical vulnerability fixes are applied not only to the
+most recent track, but to at least a couple more maintenance
+tracks.
+
+This is typically done by making the fix on the oldest and still
+relevant maintenance track, and merging it upwards to newer and
+newer maintenance tracks.
+
+For example, v2.24.1 was released to address a couple of
+[CVEs](https://cve.mitre.org/), and at the same time v2.14.6,
+v2.15.4, v2.16.6, v2.17.3, v2.18.2, v2.19.3, v2.20.2, v2.21.1,
+v2.22.2 and v2.23.1 were released.