README: briefly document that we support two Go versions

Mention that it's a general rule, since we don't jump to a newer
Go version the moment it's released, nor do we align our release cycle
with upstream Go's. As such, it's likely that a particular CUE release
will need to support three versions at once.

For example, right now we are on CUE v0.7.x which requires Go 1.20+,
but with Go 1.22.0 already being out, we are effectively supporting
CUE on three Go major releases for a bit until CUE v0.8.0 is out.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I0426bcd5a8ad59ab89824ea04694d96f8e8f7bce
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1176770
TryBot-Result: CUEcueckoo <cueckoo@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.io>

README.md

@@ -134,19 +134,23 @@ an existing set of Kubernetes configurations is available in
 
 - [`cue` Command line reference](./doc/cmd/cue.md): the `cue` command
 
+### Go release support policy
+
+As a general rule, we support the two most recent major releases of Go,
+matching Go's [security policy](https://go.dev/doc/security/policy).
+For example, if CUE v0.7.0 is released when Go's latest version is 1.21.5,
+v0.7.x including any following bugfix releases will require Go 1.20 or later.
 
 ### Contributing
 
 To contribute, please read the [Contribution Guide](CONTRIBUTING.md).
 
-
 ## Code of Conduct
 
 Guidelines for participating in CUE community spaces and a reporting process for
 handling issues can be found in the [Code of
 Conduct](https://cuelang.org/docs/contribution_guidelines/conduct).
 
-
 ## Contact
 
 You can get in touch with the cuelang community in the following ways: