doc: Add documentation for IANA well-known communities

Coded as part of #2684 and most code written while participating at
BornHack@2018.

bgp.rst: Explain what the communities does and a summarized edition of
the communities purpose.

Signed-off-by: Christoffer <netravnen@gmail.com>

doc/user/bgp.rst

@@ -1011,6 +1011,75 @@ is 4 octet long. The following format is used to define the community value.
 ``internet``
    ``internet`` represents well-known communities value 0.
 
+``graceful-shutdown``
+   ``graceful-shutdown`` represents well-known communities value
+   ``GRACEFUL_SHUTDOWN`` ``0xFFFF0000`` ``65535:0``. :rfc:`8326` implements
+   the purpose Graceful BGP Session Shutdown to reduce the amount of
+   lost traffic when taking BGP sessions down for maintainance. The use
+   of the community needs to be supported from your peers side to
+   actually have any effect.
+
+``accept-own``
+   ``accept-own`` represents well-known communities value ``ACCEPT_OWN``
+   ``0xFFFF0001`` ``65535:1``. :rfc:`7611` implements a way to signal
+   to a router to accept routes with a local nexthop address. This
+   can be the case when doing policing and having traffic having a
+   nexthop located in another VRF but still local interface to the
+   router. It is recommended to read the RFC for full details.
+
+``route-filter-translated-v4``
+   ``route-filter-translated-v4`` represents well-known communities value
+   ``ROUTE_FILTER_TRANSLATED_v4`` ``0xFFFF0002`` ``65535:2``.
+
+``route-filter-v4``
+   ``route-filter-v4`` represents well-known communities value
+   ``ROUTE_FILTER_v4`` ``0xFFFF0003`` ``65535:3``.
+
+``route-filter-translated-v6``
+   ``route-filter-translated-v6`` represents well-known communities value
+   ``ROUTE_FILTER_TRANSLATED_v6`` ``0xFFFF0004`` ``65535:4``.
+
+``route-filter-v6``
+   ``route-filter-v6`` represents well-known communities value
+   ``ROUTE_FILTER_v6`` ``0xFFFF0005`` ``65535:5``.
+
+``llgr-stale``
+   ``llgr-stale`` represents well-known communities value ``LLGR_STALE``
+   ``0xFFFF0006`` ``65535:6``.
+   Assigned and intented only for use with routers supporting the
+   Long-lived Graceful Restart Capability  as described in
+   :rfc:`draft-uttaro-idr-bgp-persistence`.
+   Routers recieving routes with this community may (depending on
+   implementation) choose allow to reject or modify routes on the
+   presence or absence of this community.
+
+``no-llgr``
+   ``no-llgr`` represents well-known communities value ``NO_LLGR``
+   ``0xFFFF0007`` ``65535:7``.
+   Assigned and intented only for use with routers supporting the
+   Long-lived Graceful Restart Capability  as described in
+   :rfc:`draft-uttaro-idr-bgp-persistence`.
+   Routers recieving routes with this community may (depending on
+   implementation) choose allow to reject or modify routes on the
+   presence or absence of this community.
+
+``accept-own-nexthop``
+   ``accept-own-nexthop`` represents well-known communities value
+   ``accept-own-nexthop`` ``0xFFFF0008`` ``65535:8``.
+   :rfc:`draft-agrewal-idr-accept-own-nexthop` describes
+   how to tag and label VPN routes to be able to send traffic between VRFs
+   via an internal layer 2 domain on the same PE device. Refer to
+   :rfc:`draft-agrewal-idr-accept-own-nexthop` for full details.
+
+``blackhole``
+   ``blackhole`` represents well-known communities value ``BLACKHOLE``
+   ``0xFFFF029A`` ``65535:666``. :rfc:`7999` documents sending prefixes to
+   EBGP peers and upstream for the purpose of blackholing traffic.
+   Prefixes tagged with the this community should normally not be
+   re-advertised from neighbors of the originating network. It is
+   recommended upon receiving prefixes tagged with this community to
+   add ``NO_EXPORT`` and ``NO_ADVERTISE``.
+
 ``no-export``
    ``no-export`` represents well-known communities value ``NO_EXPORT``
    ``0xFFFFFF01``. All routes carry this value must not be advertised to
@@ -1030,6 +1099,11 @@ is 4 octet long. The following format is used to define the community value.
    it is considered as external BGP peer, so the route will not be announced to
    the peer.
 
+``no-peer``
+   ``no-peer`` represents well-known communities value ``NOPEER``
+   ``0xFFFFFF04``  ``65535:65284``. :rfc:`3765` is used to communicate to
+   another network how the originating network want the prefix propagated.
+
 When the communities attribute is received duplicate community values in the
 attribute are ignored and value is sorted in numerical order.