Clarify/add instructions in README.md

[lukestein]

Hi. I'm an excited new user of this project but found a few things confusing in the instructions so took the liberty of trying to clarify them. Please feel free to make suggestions or just reject the pull request if you prefer, of course… I really appreciate what you've all done with this great project!

• Note both window.YTD.tweet.part0 and window.YTD.tweets.part0
• Note no progress bar on build
• Refactor Github Pages instructions
• Alternate indentation
• Clarify old filenames and old structure
• Correct arrow

[lukestein]

I note that I referenced both tweet and tweets in the docs, since some of us have old archive downloads so may encounter either

cf #47 and #65