Update CONTRIBUTING.md

fixing the doc as per discussed in the comments for pr/1824
Merge pull request #1826 from demesne/patch-2

Update CONTRIBUTING.md
Adds ability to quickly get to post’s edit page

fixes #1810

- updates frontend.single route to accept ‘edit’
as the last param

- updates controller.frontend to handle redirection
only when we would otherwise have rendered the page

- added unit tests for this behavior