Add/improve docs in README.md

[jeffpaul]

• provide external connection instructions to help with support requests like Limited connection - authorization failed: single site wordpress pulling from multi-site wordpress #357, Multisite No connection found #354, Getting: Limited connection established #327, Onboarding instructions are truly disappointing #315, "No connection found" #299, Just need to know what to put in "External Connection URL" #295, Can't Pull Content - External Connection URL Error #209, Push unavailable #41
• include note about correct registration email to utilize
• Refresh Known Caveats/Issues list Refresh Known Caveats/Issues list #361
• ^ authors aren't maintained
• ^ post content is rendered before being copied
• ^ drafts are the preferred status and can't be changed at the source site
• add table of contents to readme

Resolves #209, #295, #299, #315, #327, #357, #361.

[jeffpaul]

I've had to reference these staged docs updates several times now, so I'm going to merge these in so they're more readily available / obvious to users.