TRACK_HINTS.md: add headers
TRACK_HINTS.md: add headers

the author believes this improves readability by breaking this
information out of the flow of the exercise specific information. The
link to the Go language page has also been updated.

exercism/xgo,robphoenix/exercism-go,exercism/xgo,leenipper/xgo,robphoenix/exercism-go,leenipper/xgo