Added Boost.Build documentation

[SVN r22203]

doc/src/boost.xml

@@ -782,4 +782,5 @@
   <xi:include href="../../libs/variant/doc/variant.xml"/>
   <xi:include href="../../tools/boostbook/doc/boostbook.xml"/>
   <xi:include href="../../libs/concept_check/doc/reference/concepts.xml"/>
+  <xi:include href="../../tools/build/v2/doc/src/userman.xml"/>
 </boostbook>