Sundry changes to `README.txt`

* This is technically 4.6-dev and not 4.6.0, which might get confusing.
* Added an `About` overview.
* Pluralise `directory
* Added reference to `/js/` directory
* Switched order of post-upgrade checks, tweaked language
* Trailing slash on directory names, per the opening paragraph
Merge pull request #440 from petecooper/master

Sundry changes to `README.txt`