docs: include mirror changelog info in developer docs (#56)

Co-authored-by: Averi Kitsch <akitsch@google.com>

Author: twishabansal

DEVELOPER.md

@@ -98,7 +98,41 @@ The primary maintainers for this repository are defined in the
 
 ### Releasing
 
-The release process is automated using `release-please`.
+The release process is automated using `release-please`. It consists of an automated changelog preparation step followed by the manual merging of a Release PR.
+
+#### Automated Changelog Enrichment
+
+Before a Release PR is even created, a special workflow automatically mirrors
+relevant changelogs from the core `googleapis/genai-toolbox` dependency. This
+ensures that the release notes for this extension accurately reflect important
+upstream changes.
+
+The process is handled by the [`mirror-changelog.yml`](.github/workflows/mirror-changelog.yml) workflow:
+
+1. **Trigger:** The workflow runs automatically on pull requests created by
+   Renovate for `toolbox` version updates.
+2. **Parsing:** It reads the detailed release notes that Renovate includes in
+   the PR body.
+3. **Filtering:** These release notes are filtered to include only changes
+   relevant to this extension. The relevance is determined by a keyword (e.g.,
+   `postgres`), passed as an environment variable in the workflow file.
+4. **Changelog Injection:** The script formats the filtered entries as
+   conventional commits and injects them into the PR body within a
+   `BEGIN_COMMIT_OVERRIDE` block.
+5. **Release Please:** When the main Release PR is created, `release-please`
+   reads this override block instead of the standard `chore(deps): ...` commit
+   message, effectively mirroring the filtered upstream changelog into this
+   project's release notes.
+
+> **Note for Maintainers:** The filtering script is an automation aid, but it
+> may occasionally produce "false positives" (e.g., an internal logging change
+> that happens to contain the keyword). Before merging a `toolbox` dependency
+> PR, maintainers must **review the generated `BEGIN_COMMIT_OVERRIDE` block**
+> and manually delete any lines that are not relevant to the end-users of this
+> extension. The curated override block is the final source of truth for the
+> release changelog.
+
+#### Release Process
 
 1.  **Release PR:** When commits with conventional commit headers (e.g., `feat:`,
     `fix:`) are merged into the `main` branch, `release-please` will