Archive changelogs prior to 13.0.0

CHANGELOG.md was getting very large, so this commit:

1. Creates `changelogs/archive-$x.md` for each major version of GitLab,
   starting at GitLab 10.
2. Archives the matching EE changelogs in that directory too.
3. Documents this in changelogs/archive.md.

changelogs/README.md

@@ -8,3 +8,17 @@ To generate and validate your changelog entries:
 See [development/changelog] documentation for detailed usage.
 [development/changelog]: https://docs.gitlab.com/ee/development/changelog.html
+
+# Changelog archival
+
+The current major release is always in [CHANGELOG.md](../CHANGELOG.md) at the
+root of the repository.
+
+From GitLab 10.0.0 onwards, we store all changelog entries for a major release
+in their own file in this directory. For instance, the changelog entries for the
+10.X series of GitLab are in [archive-10.md](archive-10.md). Releases prior to
+10.0.0 have their changelogs archived in [archive.md](archive.md).
+
+Changelogs for GitLab Enterprise Edition features (all tiers) are in the
+corresponding `-ee` files, such as [CHANGELOG-EE.md](../CHANGELOG-EE.md) and
+[archive-12-ee.md](archive-12-ee.md).