Add login example to docs

[JoshuaUrrutia]

Hi, would you please add a login example to the documentation, something like:

import sbol2
SBH_USER = 'your_username'
SBH_PASSWORD = 'your_password'
sbh_stage = sbol2.PartShop('https://synbiohub.org')
sbh_stage.login(SBH_USER, SBH_PASSWORD)

Would be great, thanks!