docs: add back src/doc README.md

Author: weihanglo

src/doc/README.md

@@ -0,0 +1,7 @@
+## Contributing
+
+To contribute to Cargo's documentation. See <https://rust-lang.github.io/cargo/contrib/>.
+
+For docs specific topics, see <https://rust-lang.github.io/cargo/contrib/documentation>.
+
+For API doc, see <https://docs.rs/cargo>.