Update README.rdoc
Update README.rdoc

Updated README to remove some old pretty_print references.

wlevine/nmatrix,agisga/nmatrix,janusnic/nmatrix,janusnic/nmatrix,thisMagpie/nmatrix,johnmarinelli/nmatrix,johnmarinelli/nmatrix,thisMagpie/nmatrix,agisga/nmatrix,agisga/nmatrix,wlevine/nmatrix,janusnic/nmatrix,janusnic/nmatrix,johnmarinelli/nmatrix,wlevine/nmatrix,johnmarinelli/nmatrix,thisMagpie/nmatrix,thisMagpie/nmatrix,agisga/nmatrix,wlevine/nmatrix