added pip install from tip to docs
added pip install from tip to docs

vlas-sokolov/pyspeckit,bsipocz/pyspeckit,keflavich/pyspeckit,e-koch/pyspeckit,pyspeckit/pyspeckit,dinossimpson/pyspeckit,jpinedaf/pyspeckit,allisony/pyspeckit,keflavich/pyspeckit,jpinedaf/pyspeckit,low-sky/pyspeckit,mikelum/pyspeckit,pyspeckit/pyspeckit,bsipocz/pyspeckit,mikelum/pyspeckit,keflavich/pyspeckit-obsolete,e-koch/pyspeckit,low-sky/pyspeckit,dinossimpson/pyspeckit,vlas-sokolov/pyspeckit,allisony/pyspeckit