1. Removed trailing slash at end of rtp (runtimepath) parameter example.
The code behaves such that there should not be a trailing slash; can
very this easily by do a ":set runtimepath?"

2. Added a node that filetype plugins (which vim *assumes* will be
located in $(runtimepath)/ftplugin/[filetype]*) should have the
"ftplugin/[filetype]" part of the path excluded from the rtp parameter.

For example, the vim-instant-markdown plugin
(https://github.com/suan/vim-instant-markdown) will only work properly
if rtp is set to "after", rather than "after/ftplugin/markdown".