docs: say when a release can drop a runtime

The versioning document did not say if a minor release can raise the
minimum version of a language runtime or a database. A reader could
not tell what to expect for all of 1.x.

A minor release can raise a minimum version, but only for a version
that its own project no longer supports. A patch release cannot. One
maintainer then does not carry unsupported versions until 2.0.

Author: cardmagic

docs/versioning.md

@@ -14,6 +14,15 @@ configuration options that `README.md` and the files in `docs/` describe.
 | Minor, for example 1.1.0 | New behavior. Code that uses the public API continues to work |
 | Major, for example 2.0.0 | A change that breaks code that uses the public API |
 
+## Supported versions
+
+A minor release can raise the minimum version of Ruby, Rails, SQLite,
+PostgreSQL, or MySQL. It does this only for a version that its own project no
+longer supports, and `CHANGELOG.md` names the change. A patch release does not
+raise a minimum version.
+[Compatibility and maturity](virtual-actors.md#compatibility-and-maturity)
+lists the current minimum versions.
+
 ## Outside the promise
 
 - Classes, methods, and constants that the documentation does not describe.