doc: add example config for usrp B200 series

Change-Id: I1a80ddd9ca9b143e9c89618616f30fe3f3547d7b
diff --git a/doc/examples/osmo-trx-usrp_b200.cfg b/doc/examples/osmo-trx-usrp_b200.cfg
new file mode 100644
index 0000000..1d6656e
--- /dev/null
+++ b/doc/examples/osmo-trx-usrp_b200.cfg
@@ -0,0 +1,20 @@
+log stderr
+  logging filter all 1
+  logging color 1
+  logging print category 1
+  logging timestamp 1
+  logging level all info
+!
+line vty
+ no login
+!
+trx
+ bind-ip 127.0.0.1
+ remote-ip 127.0.0.1
+ base-port 5700
+ egprs disable
+ tx-sps 4
+ rx-sps 4
+ clock-ref external
+ chan 0
+