Update README.md and lessons page for new users

[marypiper]

Update to be more easily navigated by learners outside of the class. Link to lessons should be included and a more detailed explanation of the different sessions on the lessons page.

Use this as a template and update the link to lessons - https://github.com/hbctraining/Intro-to-ChIPseq/blob/master/README.md

It is okay if there is not an actual schedule, as long as each lesson has a time associated with it.