Use a more Symphony-ish layout for the field's settings
Update README.md

Reflects the changes made to the syntax for formatting system-values as discussed here:

https://github.com/DeuxHuitHuit/link_preview/issues/5
https://github.com/DeuxHuitHuit/link_preview/pull/4
Fixed formatting

inluding `:date:` without code-formatting shows a github emoji ;)