README: Add description for --enable-external-ell option

README

@@ -1,7 +1,7 @@
 Wireless daemon for Linux
 *************************
 
-Copyright (C) 2013-2014  Intel Corporation. All rights reserved.
+Copyright (C) 2013-2018  Intel Corporation. All rights reserved.
 
 
 Compilation and installation
@@ -48,6 +48,9 @@ It is not required to build or install Embedded Linux library. The build
 will happen when building the Wireless daemon and it will then be linked
 internally.
 
+When using --enable-external-ell build option, it is not required that the
+Embedded Linux library source code is available in the top level directory.
+
 
 Configuration and options
 =========================
@@ -114,6 +117,16 @@ setup and usability for wireless connections.
 The configuration system provides switches for optional build time features
 that can be enabled if the functionality is required:
 
+	--enable-external-ell
+
+		Enable usage of external Embedded Linux library
+
+		This allows using an externally installed Embedded Linux
+		library instead of using the internal copy of ELL.
+
+		Since the public API of Embedded Linux library  is not yet
+		stable, the usage of the internal ELL copy is preferred.
+
 	--enable-sim-hardcoded
 
 		Enable support for hard coded SIM keys