contributing update file

Author: SB-PawanN

CONTRIBUTING.md

@@ -97,8 +97,15 @@ If you're on the core team, you can release Bugsnag as follows:
     git checkout -b release/v4.x.x
     ```
 
-* Update the version number in [`setup.py`](./setup.py) and `bugsnag/notifier.py`(./bugsnag/notifier.py)
-* Update the CHANGELOG.md and README.md if necessary
+* Update the version number using the Makefile
+
+    ```
+    make bump VERSION=4.x.x
+    ```
+
+    This updates `VERSION`, `setup.py`, and `bugsnag/notifier.py` automatically.
+
+* Update the CHANGELOG.md (change `## TBD` to `## v4.x.x (YYYY-MM-DD)`) and README.md if necessary
 * Commit and open a pull request into `master`
 * Merge the PR when it's been reviewed
 * Create a release on GitHub, tagging the new version `v4.x.x`