Add info on new option to force install existing plug-ins
Add info on new option to force install existing plug-ins

Per https://github.com/dita-ot/dita-ot/pull/3120

Signed-off-by: Roger Sheen <a61a1add71dadc74037ae5e44ee30713db9a7451@infotexture.net>

dita-ot/docs,infotexture/docs,infotexture/docs