Update README.md

Add more detail to running the app.
Rendering department as HTML5 abbr (insidegov)

@maxfliri
Enabling last update date for policies module

@maxfliri
Update font size of headline to 4.8rem
Homepage text to be full width when no image

@robyoung
@pbadenski
Add link to inside government on info page

@pbadenski
'dashboard' was requested to be lowercase

@pbadenski
Using historical average for gov.uk daily traffic.

@maxfliri
@gtrogers
Updating hourly traffic for the new average

@gtrogers
@maxfliri