add contributing info to readme
add contributing info to readme

edthedev/Vim,pottereric/Vim,rufusroflpunch/Vim,VSCodeVim/Vim,AndersenJ/Vim,pottereric/Vim,Chillee/VSCodeNeovim,westim/Vim,westim/Vim,rufusroflpunch/Vim,VSCodeVim/Vim,wooyea/Vim,Chillee/Vim,edthedev/Vim,westim/Vim,Chillee/VSCodeNeovim,AndersenJ/Vim,Chillee/Vim,wooyea/Vim,Chillee/VSCodeNeovim