Add section in helpfile-menu.txt for editing pipemenus as user.

[johnraff]

At the moment the suggested method for modifying the bl-pipemenus is to edit, as root, the files in /usr/bin. As is pointed out there, this leaves the possibility of changes after a package upgrade.

I would suggest this method be advocated instead:

• Copy the file from /usr/bin to ~/bin (create directory if necessary).
• Check ~/bin comes first in $PATH.
• Edit the local file.