Add Delphix-downstream disclaimer to README

After Phase 1 of Spec 0003 lands, `delphix/sdb` is a packaging-only
downstream of `sdimitro/sdb`: `develop` tracks upstream tree-identically
plus a small overlay, and functional changes belong upstream.

Anyone landing on this repo's GitHub page from a search result, a
support ticket, or a stale link needs to know that without first
reading the spec — surface the relationship in the README header so
the upstream becomes the obvious contribution target. The notice
specifically enumerates which paths make up the overlay
(`.whitesource`, `.github/pull_request_template.md`, the notice
itself, and `debian/`) so reviewers reading the diff understand its
intentional bounds.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: prakashsurya

README.md

@@ -1,3 +1,11 @@
+> **Delphix downstream notice.** This repository is the Delphix
+> packaging downstream of [sdimitro/sdb](https://github.com/sdimitro/sdb).
+> Active development happens upstream; the `develop` branch here
+> tracks `sdimitro/develop` plus a small Delphix overlay
+> (`.whitesource`, `.github/pull_request_template.md`, this notice,
+> and `debian/` for `dh-virtualenv` packaging). Functional changes
+> belong upstream — please file PRs at sdimitro/sdb.
+
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