help: write topic for enabling and disabling plugins
Submitted by Christian Hergert
Add a help topic for enabling and disabling plugins.
Also note where to install local plugins (.local/share/gnome-builder/plugins). System installed plugins can be found in $libdir/gnome-builder/plugins/
Also add a note that if you are developing a plugin, you might want to test with PEAS_DEBUG=1 to get more information. Additionally, running Builder with -vvvv will get you more debug information.