README: Document the version of libmodbus used and update URL.

diff --git a/README b/README
index 5063069..95e7d62 100644 (file)
--- a/README
+++ b/README
@@ -550,8 +550,10 @@ Prerequisites
     <http://tangent.org/552/libmemcached.html>
 
   * libmodbus (optional)
-    Used by the `modbus' plugin to communicate with Modbus/TCP devices.
-    <https://launchpad.net/libmodbus>
+    Used by the “modbus” plugin to communicate with Modbus/TCP devices. The
+    “modbus” plugin works with version 2.0.3 of the library – due to frequent
+    API changes other versions may or may not compile cleanly.
+    <http://www.libmodbus.org/>
 
   * libmysqlclient (optional)
     Unsurprisingly used by the `mysql' plugin.