Clarify GHC support window

CONTRIBUTING.md

@@ -267,19 +267,6 @@ you push a fix of a whitespace violation, please do so in a _separate commit_. F
   support window, except Template Haskell, which would cause
   bootstrapping problems in the GHC compilation process.
 
-* Our GHC support window is five years for the Cabal library and three
-  years for cabal-install: that is, the Cabal library must be
-  buildable out-of-the-box with the dependencies that shipped with GHC
-  for at least five years.  GitHub Actions checks this, so most
-  developers submit a PR to see if their code works on all these
-  versions of GHC.  `cabal-install` must also be buildable on all
-  supported GHCs, although it does not have to be buildable
-  out-of-the-box. Instead, the `cabal-install/bootstrap.sh` script
-  must be able to download and install all of the dependencies (this
-  is also checked by CI). Also, self-upgrade to the latest version
-  (i.e. `cabal install cabal-install`) must work with all versions of
-  `cabal-install` released during the last three years.
-
 * `Cabal` has its own Prelude, in `Distribution.Compat.Prelude`,
   that provides a compatibility layer and exports some commonly
   used additional functions. Use it in all new modules.

README.md

@@ -112,6 +112,32 @@ If you are new to `cabal` and want to quickly learn the basics, check
 Got questions? Ask in [Haskell Matrix](https://matrix.to/#/#haskell:matrix.org)
 (online chat) or [Haskell Discourse](https://discourse.haskell.org).
 
+Support window
+--------------
+
+Our GHC support window is five years for both the Cabal library and
+`cabal-install`.  That is:
+
+* Cabal library must be buildable out-of-the-box against the
+  boot libraries shipped with GHC, for any GHC released in the past five
+  years from Cabal library release date.
+* `cabal-install` should be buildable with `bootstrap/bootstrap.py`
+  script, for any GHC released in the past five years from `cabal-install`
+  release date.
+* `cabal-install` should be able to drive the most recent minor version of
+  any GHCs major version released in the past five years from `cabal-install`
+  release date.
+  In this context, "drive" means `cabal-install` will work with the Cabal
+  library (usually but not always the one that came with the ghc version)
+  used to build the package.
+
+Self-upgrade to the latest version (i.e. `cabal install cabal-install`)
+must work with all versions of `cabal-install` released during the last
+three years.
+
+Cabal maintainers try to support GHC versions older than five years
+on a minimal-effort basis.
+
 Build for hacking and contributing to cabal
 -------------------------------------------