docs: deprecate this package

[shisama]

This package will be deprecated as well as request. Thank you very much for your great works.
'request-promise-native' has already been deprecated.(request/request-promise-native#57)

[coveralls]

Coverage remained the same at 100.0% when pulling 052331d on shisama:patch-1 into b4dafe4 on request:master.

[analog-nico]

Thanks a ton for taking care of this repo, too @shisama !

[jaschaio]

@shisama @analog-nico why instead of deprecating this repo not just use a maintained drop in replacement for request such as teeny-request?