Add the lost self link for site of Mime4J doc

Reported and fixed by https://github.com/dongxuwang
diff --git a/src/site/site.xml b/src/site/site.xml
index 10fc5c7..7549182 100644
--- a/src/site/site.xml
+++ b/src/site/site.xml
@@ -48,6 +48,9 @@
             <item
                 name="Server"
                 href="http://james.apache.org/documentation.html" />
+          <item
+                name="Mime4J"
+                href="http://james.apache.org/mime4j/index.html" />
             <item
                 name="jSieve"
                 href="http://james.apache.org/jsieve/index.html" />