Add more explanation to the assignment_linter()

[IndrajeetPatil]

I wanted to refer to {lintr} docs in tidyverse/style#237, but realized that it doesn't give any explanation either.

It would be good to include some documentation explaining why it might be preferable to use <- over =.

[AshesITR]

IMO the most important thing is consistency. If you commit to always use =, your code will turn out equally readable. It just happens to be that the style guide choses to prefer <- since it's more widespread among R users.

[MichaelChirico]

The style guide is where such explanation belongs IMO, and linked from our docs. Don't think there's anything we should do.