Add (for GHC) release checklist

release-checklist.md

+# For major release
+
+- Add new SPDX License list data
+
+# For release for new GHC version:
+
+- Update GHC flags in `normaliseGhcArgs`, and add the GHC version to
+  `supportedGHCVersions` (`Distribution.Simple.Program.GHC`)
+- Update `Language.Haskell.Extension` list, if there are new GHC extensions
+- Update `setupMinCabalVersionConstraint` (in `Distribution.Client.ProjectPlanning`)