Added "pp" naming convention

[SVN r23095]

more/lib_guide.htm

@@ -335,10 +335,17 @@
       <h3><a name="Directory_structure">Directory Structure</a> and Filenames</h3>
       <ul>
          <li>
-         File and directory names must contain only lowercase ASCII letters , numbers, 
+         File and directory names must contain only <b>lowercase</b> ASCII letters , numbers, 
          underscores, and a period.&nbsp; Leading character must be alphabetic. Maximum 
          length 31. Only a single period is permitted.&nbsp; These requirements ensure 
          file and directory names are relatively portable.
+         <li>
+            Files intended to be processed by a C++ compiler as part
+            of a translation unit should have <b>a three-letter
+            extension ending in &quot;pp&quot;</b>.  Other files should
+            <i>not</i> use extensions ending in &quot;pp&quot;.  This
+            convention makes it easy to identify all of the C++ source
+            in Boost.</li>
          <li>
             All libraries have at their highest level a primary directory named for the 
             particular library. See <a href="#Naming&shy;_consistency">Naming consistency</a>.