Document how to add history from old HISTFILE

[iain-palmer]

I recently changed Macbooks, and a while after installing mcfly on the new machine, I realised it would be useful to copy over my old history.

I thought it would be simple case of copying the contents of my old HISTFILE to the top of the new one (on the new machine), but unfortunately, this didn't work. I worked out that I needed to find and remove mcfly's history.db, then uninstall & reinstall mcfly. This successfully gave me all of my old history.

It would be good if there were some recommendations for moving history to a new machine in the documentation.