Compatibility definition for Project Import/Export

With the 13.0, major release, it's a good opportunity to define a more
stable backward compatibility on the Project Import feature.

doc/user/project/settings/import_export.md

@@ -44,11 +44,24 @@ Note the following:
 
 ## Version history
 
-The following table lists updates to Import/Export:
+Starting with GitLab 13.0, GitLab can import bundles that were exported from a different GitLab deployment.
+This ability is limited to two previous GitLab [minor](../../../policy/maintenance.md#versioning)
+releases, which is similar to our process for [Security Releases](../../../policy/maintenance.md#security-releases).
+
+For example:
+
+| Current version | Can import bundles exported from |
+|-----------------|----------------------------------|
+| 13.0            | 13.0, 12.10, 12.9                |
+| 13.1            | 13.1, 13.0, 12.10                |
+
+### 12.x
+
+Prior to 13.0 this was a defined compatibility table:
 
 | Exporting GitLab version   | Importing GitLab version   |
 | -------------------------- | -------------------------- |
-| 11.7 to current            | 11.7 to current            |
+| 11.7 to 13.0               | 11.7 to 13.0               |
 | 11.1 to 11.6               | 11.1 to 11.6               |
 | 10.8 to 11.0               | 10.8 to 11.0               |
 | 10.4 to 10.7               | 10.4 to 10.7               |