Improve Publishing New Releases README section

[oxarbitrage]

While making the last release (v0.1.14) i found some issues with the instructions at https://github.com/ZcashFoundation/zcash_script#publishing-new-releases that we might want to improve:

Step 2:

Run cargo release <level> to commit the release version bump (but not actually publish), <level> can be patch, minor or major

This didn't made anything for me; full logs are in #109 (comment)

We might want to remove the step. If we keep it i think it will worth noticing that all releases are of <level> = patch in general. This is the case if we are just upgrading zcashd dependency, routine release.

Step 4:

Publish a new release using cargo release --execute

This actually makes the bump happens. I think it is important here to don't have a branch with the same name as the tag we want to release.

I am not sure if that was the only cause but i had to create the tag manually in github after the release was done. Command did it for crates.io. We might need to add --push to the command.