update README

Fixes #2 (closed)

• Add note: preview site at localhost:1313/hugo/ - see https://gitlab.com/pages/hugo/issues/2#note_4728574 - no need to add --baseurl flag to the server command (tested locally)

• Move the step "Generate the website: hugo" to the end - it's a non-required step

• Add note regarding user/group site and custom domain

cc/ @axil