docs: Document new versioning scheme

Signed-off-by: Simon McVittie <smcv@collabora.com>

docs/README-versions.md

@@ -0,0 +1,60 @@
+# Versioning
+
+## Since 2.23.0
+
+SDL follows an "odd/even" versioning policy, similar to GLib, GTK, Flatpak
+and older versions of the Linux kernel:
+
+* The major version (first part) increases when backwards compatibility
+    is broken, which will happen infrequently.
+
+* If the minor version (second part) is divisible by 2
+    (for example 2.24.x, 2.26.x), this indicates a version of SDL that
+    is believed to be stable and suitable for production use.
+
+    * In stable releases, the patchlevel or micro version (third part)
+        indicates bugfix releases. Bugfix releases should not add or
+        remove ABI, so the ".0" release (for example 2.24.0) should be
+        forwards-compatible with all the bugfix releases from the
+        same cycle (for example 2.24.1).
+
+    * The minor version increases when new API or ABI is added, or when
+        other significant changes are made. Newer minor versions are
+        backwards-compatible, but not fully forwards-compatible.
+        For example, programs built against SDL 2.24.x should work fine
+        with SDL 2.26.x, but programs built against SDL 2.26.x will not
+        necessarily work with 2.24.x.
+
+* If the minor version (second part) is not divisible by 2
+    (for example 2.23.x, 2.25.x), this indicates a development prerelease
+    of SDL that is not suitable for stable software distributions.
+    Use with caution.
+
+    * The patchlevel or micro version (third part) increases with
+        each prerelease.
+
+    * Each prerelease might add new API and/or ABI.
+
+    * Prereleases are backwards-compatible with older stable branches.
+        For example, 2.25.x will be backwards-compatible with 2.24.x.
+
+    * Prereleases are not guaranteed to be backwards-compatible with
+        each other. For example, new API or ABI added in 2.25.1
+        might be removed or changed in 2.25.2.
+        If this would be a problem for you, please do not use prereleases.
+
+    * Only upgrade to a prerelease if you can guarantee that you will
+        promptly upgrade to the stable release that follows it.
+        For example, do not upgrade to 2.23.x unless you will be able to
+        upgrade to 2.24.0 when it becomes available.
+
+    * Software distributions that have a freeze policy (in particular Linux
+        distributions with a release cycle, such as Debian and Fedora)
+        should usually only package stable releases, and not prereleases.
+
+## Before 2.23.0
+
+Older versions of SDL followed a similar policy, but instead of the
+odd/even rule applying to the minor version, it applied to the patchlevel
+(micro version, third part). For example, 2.0.22 was a stable release
+and 2.0.21 was a prerelease.

docs/README.md

@@ -47,6 +47,7 @@ More documentation and FAQs are available online at [the wiki](http://wiki.libsd
 - [PSP](README-psp.md)
 - [Raspberry Pi](README-raspberrypi.md)
 - [Touch](README-touch.md)
+- [Versions](README-versions.md)
 - [WinCE](README-wince.md)
 - [Windows](README-windows.md)
 - [WinRT](README-winrt.md)