Add docs about flow spec configuration on Git's ExaBGP

docs/BGP_FLOW_SPEC.md

@@ -0,0 +1,47 @@
+### All this docs about ExaBGP 4.0 (Git master branch)
+
+Thay are not compatible with ExaBGP 3.0
+
+vim /root/announcer.py:
+
+```bash
+#!/usr/bin/python
+ 
+fl=open("/var/run/exabgp.cmd", "w")
+ 
+fl.write("announce flow route source 4.0.0.0/24 destination 127.0.0.0/24 protocol [ udp ] source-port [ =53 ] destination-port [ =80 ] packet-length [ =777 =1122 ] fragment [ is-fragment dont-fragment ] rate-limit 1024" + '\n')
+fl.flush()
+ 
+fl.close
+```
+
+Please be careful about flush and trailing '\n'!!!
+
+vim /etc/exabgp_flowspec.conf:
+```bash
+process announce-routes {
+    run /usr/bin/socat stdout pipe:/var/run/exabgp.cmd;
+    encoder json;
+}
+
+neighbor 127.0.0.1 {
+    router-id 1.2.3.4;
+    local-address 127.0.0.1;
+    local-as 1;
+    peer-as 1;
+    group-updates false;
+
+  family {
+        ipv4 flow;
+    }
+    api {
+        processes [ anounce-routes ];
+    }
+}
+
+```
+
+Run it:
+```bash
+env  exabgp.api.file=/tmp/exabgp.cmd exabgp.daemon.user=root exabgp.daemon.daemonize=false exabgp.daemon.pid=/var/run/exabgp.pid exabgp.log.destination=/var/log/exabgp.log sbin/exabgp --debug /etc/exabgp_flowspec.conf 
+```