Document option -append

[fievel]

For my specific use (launching grep like command (ripgrep) on a complex source hierarchy), I needed to be able to run several AsyncRun without clearing the quickfix between runs. I searched the documentation and found nothing, so I wanted to hack the code of the plugin. I was very happy when I found that a -append option existed and did exactly what I wanted.

As this might be useful to other users as well, I think the documentation should be updated in order to make your user aware of this simple yet useful option.