- Add Use pure to uses.xml

Approved by:	pawel (mentor)

en_US.ISO8859-1/books/porters-handbook/uses.xml

@@ -164,6 +164,17 @@
     dependencies.</entry>
 </row>
 
+<row>
+  <entry><literal>pure</literal></entry>
+  <entry>(none), <literal>ffi</literal></entry>
+  <entry>Implies that the port uses <filename
+      role="package">lang/pure</filename> in one way or another,
+    but largely is used for building related pure ports.
+    With the <literal>ffi</literal> argument, it implies <filename
+      role="package">devel/pure-ffi</filename>
+    as a run-time dependency.</entry>
+</row>
+
 <row>
   <entry><literal>qmail</literal></entry>
   <entry>(none), <literal>build</literal>, <literal>run</literal>,