Add npm install method to homepage

[Epskampie]

I feel this is important as it is the way most users will want to use definitelytyped nowadays.

This not being there actually made me think definitelytyped was something different from @types when I was learning typescript... :-)

I stuck as much as possible to the DT readme and the naming in the official TS handbook.

[vvakame]

sorry for late! thanks

[Epskampie]

Cool, no worries, thanks for merge!