Re-write of learning disabilities paragraph

Make the tone a bit more formal in keeping with the rest of the document.
Removed superfluous final sentence which is covered in next paragraph titled "Why we do this"
Update how-users-read.md

Clarified plain English section, tidied up grammar and fixed style points.
Merge pull request #292 from alphagov/copy-change-how-users-read

Re-write of learning disabilities paragraph