Added stand-alone headers guideline.

[SVN r11774]

header.htm

@@ -39,6 +39,10 @@ these are also reasonable guidelines for general use.
     to older approaches such as adding funny prefixes to global names. 
     Libraries which are designed to work well with other Boost libraries should
     be placed namespace <tt>boost</tt>.</li>
+
+  <li>Make sure that a translation unit consisting of just the
+  contents of the header file will compile successfully.
+
   <li>Place the header file in a sub-directory to prevent conflict with
     identically named header files in other libraries.&nbsp; The parent
     directory is added to the compiler's include search path.&nbsp; Then both