Clarification on which hooks; warning on initial gemfiles

Added a warning and a workaround to install gems to global gemset.

There are two files:

    ./hooks/after_install_auto_gem
    ./hooks/after_install_codesign

that aren't documented here. However, it's quite useful to have custom after_install scripts. Specifically to pre-install some useful gems, since .rvm/gemsets/default.gems and .rvm/gemsets/global.gems are usually overriten during `rvm update`.