Add a stub manual page for iflib(9).

Currently 'man -k iflib' would find you the right pages for iflib
documentation, namely iflibdd(9) and iflibdi(9) but 'man iflib' would leave
you in the dark. This allows both approaches to find the relevant
documentation.

Reviewed by:	kmacy, shurd
Sponsored by:	iXsystems, Inc.
Differential Revision:	https://reviews.freebsd.org/D15219

share/man/man9/Makefile

@@ -163,6 +163,7 @@ MAN=	accept_filter.9 \
 	ieee80211_regdomain.9 \
 	ieee80211_scan.9 \
 	ieee80211_vap.9 \
+	iflib.9 \
 	iflibdd.9 \
 	iflibdi.9 \
 	iflibtxrx.9 \

share/man/man9/iflib.9

@@ -0,0 +1,41 @@
+.\" $FreeBSD$
+.Dd May 3, 2018
+.Dt IFLIB 9
+.Os
+.Sh NAME
+.Nm iflib
+.Nd Network Interface Driver Framework
+.Sh DESCRIPTION
+.Nm
+is a framework for writing network interface drivers for FreeBSD.
+It is designed to remove a large amount of the boilerplate that is often
+needed for modern network interface devices, allowing driver authors to
+focus on the specific code needed for their hardware.
+.Pp
+There are three logical components to
+.Nm
+each of which is described in its own manual page.
+These are:
+.Bl -tag -width ".Xr iflibtxrx 9"
+.It Xr iflibdi 9
+Device-independent functions, used to integrate
+.Nm
+into the rest of the
+.Fx
+networking stack.
+.It Xr iflibdd 9
+Device-dependent functions, used when writing new
+.Nm
+based drivers.
+.It Xr iflibtxrx 9
+Device-dependent transmit and receive functions, used when writing new
+.Nm
+based drivers.
+.Sh SEE ALSO
+.Xr iflibdd 9 ,
+.Xr iflibdi 9 ,
+.Xr iflibtxrx 9 ,
+.Xr ifnet 9
+.Sh AUTHORS
+.An Benno Rice Aq Mt benno@FreeBSD.org
+