I just downloaded the Rails vim plugin from here.
One of the file that it contain is a doc file that will be extracted to
How do I open the content of this help file in
:help rails to no avail.
After installing a new help file into ~/.vim/doc, you need to rebuild the help tags file:
garyjohn's answer is right, but I just want to mention that pathogen provides an easy way to run helptags for each plugin (note the case):