Add SIP guide about how to implement DNS SRV failover (from trac FAQ).

docs/source/specific-guides/sip/dns_failover.rst

@@ -0,0 +1,16 @@
+Implement DNS SRV failover
+=========================================
+Our DNS SRV failover support is only limited to TCP (or TLS)
+:cpp:any:`connect()` failure, which in this case pjsip will automatically
+retries the next server. But even then, there is no mechanism to flag that
+a server has been failing, which means that the next request may try
+the same server again and triggering the failover again.
+
+What we've been suggesting is to implement the failover mechanism in the
+application layer. In this case, the application queries the list of available
+servers either with :cpp:any:`gethostbyname()`, DNS SRV, or by other means.
+It then specifies which server to use by putting the IP address as
+proxy parameter (i.e. Route header) in the account config. The mechanism to
+test the wellness of a server and when to initiate the failover is totally
+controlled by the application. The application can change which server to
+use by changing the account proxy setting with :cpp:any:`pjsua_acc_modify()`.