Minor changes to quickstart guide in README

- Clarify the distinction between *repository settings* and *user settings*.
- Standardize capitalization of *GitHub* and *Read the Docs*.
- Add periods to the end of each list item.
- Change *quick start* to *quickstart*.
- Change *login* to *log in* (noun vs. verb distinction).
- Standardize use of quotation marks surrounding "click targets".