The cunfiguration chapter does not say anything about E1 trunk
configurations, lets add a subsection that explains how an E1
trunk is added.
Change-Id: I4059771df0f115242398d1af499c837005a7dabc
The trunk 0 was reserved for rtpbridge in earlier versions of osmo-mgw,
this was due to internal technical limitations which no longer exist.
Change-Id: Ic322f7dcf28db827187a4537e2634a887bd7b13b