Modified release instructions

I added the instruction to update the version in `package.json`. Also, do we need to manually change the version in `_frontend.py` or does that happen automatically as a consequence of updating the version of `_version.py`?

Author: cornhundred

README.md

@@ -79,6 +79,7 @@ of those flags here.
 - Add tests
 - Ensure tests pass locally and on CI. Check that the coverage is reasonable.
 - Make a release commit, where you remove the `, 'dev'` entry in `_version.py`.
+- Update the version in package.json
 - Relase the npm packages:
   ```bash
   npm login