docs: networking: Convert bridge.txt to rst

The kernel documentation is now restructured text. Convert the Ethernet
Bridge documentation and include it in the toplevel kernel
documentation.

 - Fix heading adornments.
 - Add license identifier.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/bridge.rst

@@ -0,0 +1,21 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+Ethernet Bridging
+=================
+
+In order to use the Ethernet bridging functionality, you'll need the
+userspace tools.
+
+Documentation for Linux bridging is on:
+   http://www.linuxfoundation.org/collaborate/workgroups/networking/bridge
+
+The bridge-utilities are maintained at:
+   git://git.kernel.org/pub/scm/linux/kernel/git/shemminger/bridge-utils.git
+
+Additionally, the iproute2 utilities can be used to configure
+bridge devices.
+
+If you still have questions, don't hesitate to post to the mailing list 
+(more info https://lists.linux-foundation.org/mailman/listinfo/bridge).
+