go.talks/present: document the .extensions Related: remove stale mention of -template flag.
go.talks/present: document the .extensions
Related: remove stale mention of -template flag.

R=golang-dev, campoy, adg
CC=golang-dev
https://golang.org/cl/6851046

Karthikvb/talks,Karthikvb/talks,Karthikvb/talks,Karthikvb/talks,Karthikvb/talks,Karthikvb/talks