release instructions: make clear what to do before signing the release candidate

It's key to do at least these in the sources that will go into the release _before_ starting the signing/build release process:
- npm: release version in package.json
- up to date CHANGELOG

But it's not very clear from the [document](https://github.com/apache/openwhisk-release/blob/master/docs/release_instructions.md)... could be highlighted more.