Update README with instructions on how to generate API documentation.

Fix reference path to NUnit for documentation generation task.
Fixes [AMQNET-302]. (See https://issues.apache.org/activemq/browse/AMQNET-302)

README.txt

@@ -19,9 +19,21 @@ To run the unit tests you need to run an Apache ActiveMQ Broker first then run:
 
   nant test
 
+The NMS documentation can be generated into three different formats using
+Microsoft's Sandcastle open source product. The Sandcastle Styles project
+was used to enhance the output generated from the current release of Sandcastle.
+
+The Sandcastle project is located here:
+
+http://sandcastle.codeplex.com/
+
+The Sandcastle Styles project is located here:
+
+http://sandcastlestyles.codeplex.com/
+
 To generate the documentation, run:
 
-  nant doc
+  nant sandcastle-all
 
 =======================================================================
 Building With Visual Studio 2008

nant-common.xml

@@ -545,7 +545,7 @@
         <property name="sandcastle.style" value="vs2005" unless="${property::exists('sandcastle.style')}" />
         <property name="documentation.dir" value="${build.bin.dir}" />
         <property name="bin.intern.dir" value="${build.bin.dir}" />
-        <property name="bin.extern.dir" value="${basedir}\vendor\NUnit\net-2.0" />
+        <property name="bin.extern.dir" value="${basedir}\lib\NUnit\net-2.0" />
         <property name="sandcastle.dir" value="${environment::get-variable('DXROOT')}" />
         <property name="sandcastle.workingdir" value="${build.dir}\doc\${sandcastle.style}" />
         <property name="sandcastle.output.dir" value="${sandcastle.workingdir}\Output" />