Update README.md

- fix a small typo
- restructure sentence about responding to X-PJAX headers
Merge pull request #294 from tylerwolff/master

Update README.md with some text fixes
Don't default fragment to null
pjax 1.7.1
Fix autofocus in FF

/cc @jonrohan
Clear out any focused elements before inserting new page contents
Merge pull request #300 from defunkt/refocus

Improve focus emulation
pjax 1.7.2