librdmacm: Document to set accept_local sysctl for loopback

For loopback connections between ports on a single system to work,
accept_local sysctl must be set to 1.  Add this to the readme.

Signed-off-by: Sean Hefty <[email protected]>

Author: shefty

README

@@ -48,3 +48,14 @@ Using multiple interfaces
 	address carried in the ARP request.  This causes the RDMA stack
 	to incorrectly map the remote IP address to the wrong RDMA
 	device.
+
+Using loopback
+	The librdmacm relies on ARP to resolve IP address to RDMA
+	addresses.  To support loopback connections between different
+	ports on the same system, ARP must be enabled for local
+	resolution:
+
+	sysctl net.ipv4.conf.all.accept_local=1
+
+	Without this setting, loopback connections may timeout
+	during address resolution.