Bump version to v4.2.0 and update notes on creating new releases

[tompollard]

This is a minor change that:

• Bumps the version number to 4.1.3
• Updates the notes on creating new releases

After this change we will need to:

• Update the changelog
• Update the documentation if necessary
• Create a new release at: https://github.com/MIT-LCP/wfdb-python/releases/new

[bemoody]

I'm okay with cutting a new release here - I think addressing the FLAC bugs is unfortunately still going to take some time.

Since write_dir was added in pull #492, this should be a minor version bump (4.2.0).

The notes here don't mention how to update wfdb.readthedocs.io. I don't remember how that's done.

[tompollard]

@bemoody I bumped to v4.2.0 as suggested.

The notes here don't mention how to update wfdb.readthedocs.io. I don't remember how that's done.

I can look into this, but would prefer to do in a new pull request.