Add "Best practice" section to user guide

[helgoboss]

Some first material that I wrote once in a forum post (about where to put instances):

This is completely up to you, there are no strict rules and it all depends on what you want. Let me try to come up with some rules of thumb to not leave you totally lost (these will probably need some refinement):

1. You need at least one ReaLearn instance per controller.
2. For mappings that shall be available in any existing or new project automatically, create a ReaLearn instance on the monitoring FX chain (again, per controller).
3. For in-project mappings which control arbitrary parameters on arbitrary tracks, you are totally free to choose where to put ReaLearn, there shouldn't be any differences in behavior. Putting it on the master FX chain is a good choice if you want it out of your way and usually not be visible. However, in order to be reminded that I use ReaLearn, I usually add some track at the very top and put all ReaLearn instances on its FX chain.
4. Let's say you have a VST instrument that you want to activate/deactivate for live playing by arming/disarming the corresponding track. And now you want to use ReaLearn as a kind of "Insert" effect for controlling parameters of that VST instrument (or other track-local parameters such as the track volume), only when it's active. Then put ReaLearn exactly on that track, somewhere above your VST instrument and select MIDI control input "". (That way your VST instrument won't receive MIDI that's already processed by ReaLearn, which is cool. If you want ReaLearn to only receive MIDI messages that originate from live playing and ignore MIDI that originates from track MIDI items, put it on the input FX chain of that track.)