[src/doc] Add README.md

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+# Cargo Documentation
+
+NOTE: Cargo documentation is under migration to mdBook-based structure. All the
+`*.md` files here shall be kept in sync with the `*.md` files under `book/src/`.
+See `MIGRATION_MAP` file here and <https://github.com/rust-lang/cargo/pull/4453>
+for details.