rust-readme: mentioned that format check is enforced by CI

rust/README.rst

@@ -119,7 +119,9 b' We use ``rustfmt`` to keep the code form'
 using the nightly version because it has been stable enough and provides
 comment folding.
 
-To format the entire Rust workspace::
+Our CI enforces that the code does not need reformatting. Before
+submitting your changes, please format the entire Rust workspace by running::
+
 
   $ cargo +nightly fmt