Add example to the documentation of modbus_read_registers

doc/modbus_read_registers.txt

@@ -36,6 +36,37 @@ EMBMDATA::
 Too many registers requested
 
 
+EXAMPLE
+-------
+[source,c]
+-------------------
+modbus_t *ctx;
+uint16_t tab_reg[64];
+int rc;
+int i;
+
+ctx = modbus_new_tcp("127.0.0.1", 1502);
+if (modbus_connect(ctx) == -1) {
+    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
+    modbus_free(ctx);
+    return -1;
+}
+
+rc = modbus_read_registers(ctx, 0, 10, tab_reg);
+if (rc == -1) {
+    fprintf(stderr, "%s\n", modbus_strerror(errno));
+    return -1;
+}
+
+for (i=0; i < rc; i++) {
+    printf("reg[%d]=%d (0x%X)\n", i, tab_reg[i], tab_reg[i]);
+}
+
+modbus_close(ctx);
+modbus_free(ctx);
+-------------------
+
+
 SEE ALSO
 --------
 linkmb:modbus_write_register[3]