Cover package registration via twine

[ncoghlan]

This change presents twine as a CLI option for registration as well.

It also moves the note about manual .pypirc creation to the section
on creating an account, as that is needed for twine register to work.

[ncoghlan]

@dstufft @mgedmin noted twine register wasn't covered, but in reviewing the PR at #217 I noticed several other problems with the relevant section.

This PR aims to address those as well.

[pfmoore]

"in order to protect"

[dstufft]

I'm not that great at documentation, but this is slightly wrong. It may use a plaintext HTTP connection, or it may use an unverified HTTPS connection, or it may use a verified HTTPS connection based on what version of Python you're using and/or what patches have been applied to your Python by whomever you're getting it from.

It may be over complicated to go into that much detail though and it may be preferable to be technically wrong, but simpler and safer in this documentation.

[dstufft]

One comment to consider, otherwise looks good to me.

For the record, there's no need to ever use an explicit register step in Warehouse, even in legacy PyPI it's only needed once to register the project to your name, in Warehouse you don't even need it that one time.

[ncoghlan]

Thanks folks - 2nd commit addressed @dstufft's and @pfmoore's comments, so I've merged this and any further changes can go in another PR.

Thanks again for the prompt to address it, @mgedmin.

[mgedmin]

🍰