Add version matrix to README.md

Author: LMBishop

README.md

@@ -66,3 +66,45 @@ following subprojects depend on this
 * `/forge`: specific implementation for Forge servers
 
 Note that this is a server only mod, and will not work on clients.
+
+## Version matrix
+
+The following table documents mod compatibility for older versions of Minecraft and their platforms. 
+Any versions prior to 1.12.1 are backports.
+
+If a Minecraft version is not listed here, then no version of the mod exists for it.
+
+All version branches will follow the name `minecraft/<version>`.
+
+| Minecraft | Java | Bukkit | Forge | Fabric | Links                                                                   |
+|-----------|------|--------|-------|--------|-------------------------------------------------------------------------|
+| 1.21.1    | 21   | ✅      | ✅     | ✅      | (master)                                                                |
+| 1.18.2    | 17   | ❌      | ❌     | ✅      | [Branch](https://github.com/CSSUoB/CSS-Minecraft/tree/minecraft/1.18.2) |
+| 1.12.2    | 8    | ❌      | ✅     | ❌      | [Branch](https://github.com/CSSUoB/CSS-Minecraft/tree/minecraft/1.12.2) |
+
+**Never merge `minecraft/*` branches into master.** Build features/fixes in master and cherry-pick backwards.
+
+### Upgrading to future versions
+
+The `master` branch should always target the latest version. 
+Before upgrading, create a new release branch for the current version using the naming
+scheme `minecraft/<version>`.
+
+Then, make the necessary changes to upgrade Minecraft version. Bukkit / Spigot / Paper 
+has a stable enough API where not many changes will be needed (if any at all), but
+other platforms will likely break.
+
+Once changes are done, update the version matrix and open a new PR to `master`.
+
+### Backporting to older versions
+
+This mod was originally made for Minecraft 1.21, thus
+it will require backporting to work on older modpacks.
+
+Create a branch from the nearest Minecraft version and name it `minecraft/<version>`. 
+You may be required to change the Java version, or upgrade/downgrade Gradle. 
+It should be noted that Fabric does not exist prior to Minecraft 1.14.
+
+Once finished, push the branch to GitHub and update this version matrix with the platform
+and version you have backported.
+