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)

diff --git a/nant-common.xml b/nant-common.xml
index 02c4d14..bf7a6ae 100644
--- a/nant-common.xml
+++ b/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" />