docs: add comment regarding pip/pip3 differences
docs: add comment regarding pip/pip3 differences

bastimeyer/streamlink,beardypig/streamlink,chhe/streamlink,streamlink/streamlink,gravyboat/streamlink,bastimeyer/streamlink,melmorabity/streamlink,beardypig/streamlink,melmorabity/streamlink,gravyboat/streamlink,streamlink/streamlink,chhe/streamlink