add information about the tex4ht requirement for the docs

SVN-Revision: 6384

README

@@ -14,7 +14,7 @@ You can use scripts/flash.sh for remotely updating your embedded system
 via tftp.
 
 The OpenWrt system is documented in docs/. You will need a LaTeX distribution
-to build the documentation. Type make -C docs/ to build the documentation.
+and the tex4ht package to build the documentation. Type make -C docs/ to build it.
 
 Building your own firmware you need to have access to a Linux, BSD or MacOSX system.
 Cygwin will not be supported because of the lack of case sensitiveness.