Don't claim that Boost.Pool is required, because it isn't

Replace "required" with "recommended" in the build requirements section.

Author: vadz

README

@@ -23,7 +23,8 @@ In order to build xmlwrapp, you need libxml2 version 2.4.28 or newer. When
 building with XSLT support, libxslt 1.1.6 or newer is required. Both libraries
 are available from http://xmlsoft.org.
 
-Header-only Boost Pool library is also required for building xmlwrapp.
+Header-only Boost Pool library is recommended, for better performance, but not
+required.
 
 To run tests, Boost.Iostreams and Boost.Test libraries are also needed.