doc: add README for the BgpIpv6Basic example (IPv6 support, phase 6)

Author: avarga

examples/bgpv4/BgpIpv6Basic/README.md

@@ -0,0 +1,29 @@
+# BgpIpv6Basic — BGP over IPv6 (MP-BGP, RFC 4760)
+
+A minimal IPv6 EBGP example: two single-router autonomous systems peer over IPv6 and each
+advertises the IPv6 network behind it.
+
+```
+hostA --- rA (AS 65001) === rB (AS 65002) --- hostB
+        2001:db8:0:10::/64   2001:db8:0:1::/64   2001:db8:0:20::/64
+```
+
+What it demonstrates / verifies:
+
+- A BGP session is established over **IPv6 TCP** (the `Bgp` module runs with
+  `addressFamily = "ipv6"` and drives the IPv6 routing table).
+- IPv6 reachability is exchanged using the **`MP_REACH_NLRI`** path attribute
+  (AFI=2 / SAFI=1), per RFC 4760 — rA advertises `2001:db8:0:10::/64`, rB advertises
+  `2001:db8:0:20::/64`, each via a `<Network>` element in `BgpConfig6.xml`.
+- The learned routes are actually **installed and used**: the configurator adds only
+  on-link routes (`addStaticRoutes = false` + manual `<route>` entries), so the inter-AS
+  routes can come *only* from BGP. `hostA` pinging `hostB` therefore succeeds only once
+  BGP has converged.
+
+Notes:
+
+- The BGP Identifier is a 4-octet router id even for IPv6 (RFC 4271/6286); it is set
+  explicitly via `bgp.routerId` since an IPv6 router has no IPv4 address to derive it from.
+- The manual on-link `<route>` entries are a workaround for a configurator limitation
+  (`addDirectRoutes` is only applied when `addStaticRoutes` is true); see
+  `plan/pending/configurator-decouple-direct-routes.md`.