Added general RPL border router readme

examples/rpl-border-router/README.md

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+This is the Contiki-NG border router. It supports two main modes of operation:
+embedded and native. In both cases, the border router runs a simple Web server
+that exposes a list of currently connected nodes via HTTP.
+
+See https://github.com/contiki-ng/contiki-ng/wiki/Tutorial:-RPL-border-router
+
+# Embedded border router
+
+The embedded border router runs on a node. It is connected to the host via SLIP.
+The host simply runs a tun gateway (`tunslip6`). To use, program a node, and
+then start `tunslip6` on the host via the make command `connect-router`.
+See embedded/README.md for more.
+
+# Native border router
+
+The native border router runs directly at the host. The node simply runs a
+SLIP-radio interface (`examples/slip-radio`). The host, on the other hand, runs
+a full 6LoWPAN stack.
+See native/README.md for more.
+
+# RPL node
+
+As RPL node, you may use any Contiki-NG example with RPL enabled, but which
+does not start its own DAG (as this is the responsibility of the border router).
+For instance `examples/hello-world` or `examples-coap` are great starting
+points. This is not intended to run with `examples/rpl-udp` however, as this
+examples builds its own stand-alone, border-router-free RPL network.