Correct prerequisites OS version in installer ReadMe to 10.6

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2608 f4f36d40-9719-0410-9868-dde021a23eda
Add a help message of install.rb for `package --config-file=`

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2609 f4f36d40-9719-0410-9868-dde021a23eda