Clarification on which hooks can be suffixed

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`.
Add warning that files are overwriten on update

Add warning: default.gems and global.gems files are usually overriten during `rvm update`
default.gems/global.gems are overwritten: warn

Warning and work around default.gems/global.gems from being overwritten.
Gem flags documented

--without-gems, --with-default-gems, --with-gems flags documentation
added
Merge pull request #212 from pirj/gemset-doc-improvment

Clarification on hooks; warning on initial gemsets; gemset doc improved