Minor README touchups
Minor README touchups

After going through the readme on a fresh machine, these are some recommendations.
mrakitin/sirepo,mrakitin/sirepo,mkeilman/sirepo,radiasoft/sirepo,mrakitin/sirepo,mkeilman/sirepo,mkeilman/sirepo,radiasoft/sirepo,mkeilman/sirepo,radiasoft/sirepo,radiasoft/sirepo,mrakitin/sirepo,radiasoft/sirepo