Add Interactive Service developer documentation

Add Interactive Service developer documentation

The OpenVPN Interactive Service documentation from
https://community.openvpn.net/openvpn/wiki/OpenVPNInteractiveService was
upgraded with a description of the client-service communication flow,
service registry configuration, and non-default instance installation.

Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20180419112313.1013-1-simon@rozman.si>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16794.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>