Update README release commands to sign release tags

[sbesson]

Minor improvements from my experience releasing omero-py 5.7.0.

A second minor is that if the previous dev version does not match the expected release version (in my case 5.6.3.dev0 was released as 5.7.0), the default command will obviously create an undesired tag/commit. Options are either:

• to review the current version and have a preliminary bump of bumpversion --no-tag [major|minor|patch]
• to override the content of .bumpversion.cfg using `bumpversion release --current-version/--new-version
  No strong preference either way but if people have a inclination towards either of these options, I can certainly add a sentence to the README as part of this PR

[joshmoore]

Doh! Somewhere along the line we lost this:

(base) /opt $grep sign */.bumpversion.cfg
ome-zarr-py/.bumpversion.cfg:sign_tags = True
omero-dropbox/.bumpversion.cfg:sign_tags = True
omero-figure/.bumpversion.cfg:sign_tags = True
omero-signup/.bumpversion.cfg:sign_tags = True

[sbesson]

Updated to set the defaults in the configuration file instead.

[joshmoore]

Thanks. I've blindly been assuming I've had this active everywhere.