complete copy-edit, clean up of GUIDE.md
complete copy-edit, clean up of GUIDE.md

I decided to read the GUIDE.md by editing it.
I thoroughly cleaned up the file, removing extraneous words,
fixing line lengths, making grammar fixes, clarifying unclear bits,
improving formatting, and generally improving anywhere I noticed room for improvement.
sinelaw/stack,anton-dessiatov/stack,keent/stack,juhp/stack,luigy/stack,Fuuzetsu/stack,harendra-kumar/stack,Heather/stack,Andrew-the-Ubergeek/stack,mathhun/stack,tedkornish/stack,athanclark/stack,harendra-kumar/stack,AndrewRademacher/stack,supermario/stack,AndrewRademacher/stack,supermario/stack,rubik/stack,sinelaw/stack,sjakobi/stack,narrative/stack,athanclark/stack,vigoo/stack,sjakobi/stack,robstewart57/stack,tedkornish/stack,kairuku/stack,rvion/stack,mrkkrp/stack,rrnewton/stack,deech/stack,anton-dessiatov/stack,AndreasPK/stack,luigy/stack,phadej/stack,rubik/stack,MichielDerhaeg/stack,phadej/stack,mrkkrp/stack,martin-kolinek/stack,MichielDerhaeg/stack,lukexi/stack,fizruk/stack,meiersi-11ce/stack,Blaisorblade/stack,fizruk/stack,Heather/stack,cabrera/stack,robstewart57/stack,meiersi-11ce/stack,vigoo/stack,martin-kolinek/stack,rrnewton/stack,rvion/stack,cabrera/stack,mathhun/stack,Fuuzetsu/stack,kairuku/stack,AndreasPK/stack,achernyak/stack,juhp/stack,adinapoli/stack,lukexi/stack,narrative/stack,deech/stack,achernyak/stack,adinapoli/stack,Blaisorblade/stack,Andrew-the-Ubergeek/stack,keent/stack