docs: Add version compatibility matrix

Explain which tagged release, and which Git branch, is compatible with
which Open edX/Tutor release.

README.md

@@ -5,6 +5,25 @@ This is an **experimental** plugin for
 [Tutor](https://docs.tutor.overhang.io) that enables the [user retirement 
 feature](https://edx.readthedocs.io/projects/edx-installing-configuring-and-running/en/latest/configuration/user_retire/index.html) in Open edX.
 
+Version compatibility matrix
+----------------------------
+
+You must install a supported release of this plugin to match the Open
+edX and Tutor version you are deploying. If you are installing this
+plugin from a branch in this Git repository, you must select the
+appropriate one:
+
+| Open edX release | Tutor version     | Plugin branch | Plugin release |
+|------------------|-------------------|---------------|----------------|
+| Lilac            | `>=12.0, <13`     | Not supported | Not supported  |
+| Maple            | `>=13.2, <14`[^1] | `maple`       | 0.1.x          |
+| Nutmeg           | `>=14.0, <15`     | `main`        | 1.x.x          |
+
+[^1]: For Open edX Maple and Tutor 13, you must run version 13.2.0 or
+   later. That is because this plugin uses the Tutor v1 plugin API,
+   [which was introduced with that
+   release](https://github.com/overhangio/tutor/blob/master/CHANGELOG.md#v1320-2022-04-24).
+
 Installation
 ------------