added submodule tip
added submodule tip

hadley/docs,matsen/docs,nhumrich/docs,ryanbauman/docs,kordless/docs,tbenade/docs,nhumrich/docs,ramachandra3206/docs,hadley/docs,yoshuawuyts/docs,ryanbauman/docs,Starefossen/docs-1,yoshuawuyts/docs,matsen/docs,kordless/docs,ramachandra3206/docs,Starefossen/docs-1,tbenade/docs