chore: Add README for verified sources fetcher (matter-labs#2829)

## What ❔

Adds README for verified sources fetcher

## Checklist

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- [ ] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [ ] Documentation comments have been added / updated.
- [ ] Code has been formatted via `zk fmt` and `zk lint`.

core/bin/verified_sources_fetcher/README.md

@@ -0,0 +1,4 @@
+# Verified sources fetcher
+
+This tool downloads verified contract sources from SQL database from `contract_verification_requests` table. Then it
+saves sources and compilation settings to files.