DOC: Promote migration notice to README.md so GitHub landing page shows archived status

The LabelErodeDilate module is ingested into ITK proper at
Modules/Filtering/LabelErodeDilate as of upstream commit 690015cae2
(PR #6171). Renaming the original README.rst to info.rst and adding
a top-level README.md migration notice ensures GitHub's README
precedence picks the migration notice on the landing page rather
than the (now stale) module description.

Tracking: InsightSoftwareConsortium/ITK#6160

Author: hjmjohnson

README.md

@@ -0,0 +1,23 @@
+# Migrated to ITK main
+
+As of 2026-04-30, the `LabelErodeDilate` module has been ingested into
+the main ITK source tree. The authoritative location is:
+
+  https://github.com/InsightSoftwareConsortium/ITK/tree/main/Modules/Filtering/LabelErodeDilate
+
+See `Modules/Filtering/LabelErodeDilate/README.md` (or any in-tree
+documentation under that path) for details on what moved and what
+remains in this archived repo.
+
+This repository is retained read-only for historical reference (deep
+git history, paper material, example assets not migrated to ITK). It
+will be marked ARCHIVED immediately after this commit lands.
+
+The original module description that previously appeared at this
+repository's landing page has been preserved as `info.rst` in the
+repository root.
+
+Related:
+- ITK ingest PR: InsightSoftwareConsortium/ITK#6171
+- ITK ingest merge commit: InsightSoftwareConsortium/ITK@690015cae2
+- Tracking issue: InsightSoftwareConsortium/ITK#6160