Add note, that a merge commit after push is necessary

Author: flip1995

CONTRIBUTING.md

@@ -178,6 +178,15 @@ to be run inside the `rust` directory):
     _Note:_ This will directly push to the remote repository. You can also push
     to your local copy by replacing the remote address with `/path/to/rust-clippy`
     directory.
+
+    _Note:_ Most of the time you have to create a merge commit in the
+    `rust-clippy` repo (this has to be done in the Clippy repo, not in the
+    rust-copy of Clippy):
+    ```bash
+    git checkout sync-from-rust
+    git fetch upstream
+    git merge upstream/master
+    ```
 3. Open a PR to `rust-lang/rust-clippy` and wait for it to get merged (to
    accelerate the process ping the `@rust-lang/clippy` team in your PR and/or
    ~~annoy~~ ask them in the [Discord] channel.)