readme.md edit

-missing comma
-use of "a" preceding an acronym with phonetic vowel
Merge pull request #4730 from sdtsui/patch-1

readme.md edit
Update README.md
Revert punctuation change in README.md
Removed double link, moved contributing ref.

- Contributing ref link was being gratuitously double-linked to, nixed that.
- Changed grammar which is ambiguous between US/UK colloquialism.
- Moved contributing/troubleshooting reference to "installing from git" heading rather than "quickstart" subheading
- Standardised 2-linebreaks between Markdown headings
Merge branch 'master' of https://github.com/TryGhost/Ghost