docs: add a simple example to use configuration file

Change-Id: I013f03779508bc3ee5bb378c357a5c94b40b74a0

README.md

@@ -19,9 +19,19 @@ $ rebar3 eunit
 
 == Running ==
 
-Once osmo_dia2gsup is built, you can start it this way:
+Once osmo\_dia2gsup is built, you can start it this way:
 
 $ rebar3 shell
 
 In the erlang shell:
+```
 1> osmo_dia2gsup:start().
+```
+
+== Configuration ==
+
+$ rebar3 shell --config ./examples/sys.config
+```
+1> osmo_dia2gsup:start().
+```
+

examples/sys.config

@@ -0,0 +1,12 @@
+[{osmo_dia2gsup, [
+        {hlr_ip, "127.0.0.1"},
+        {hlr_port, 4222},
+
+        {diameter_ip, "127.0.0.4"},
+        {diameter_port, 3868},
+        {diameter_proto, sctp},
+
+        {origin_host, "hss.localdomain"},
+        {origin_realm, "localdomain"},
+        {vendor_id, 0}
+]}].