doc: small doxygen update

I was merging change this every time before generation of
the doxygen developer docs (https://dev.visucore.com/bitcoin/doxygen/)
but there's no reason why it can't just be merged upstream.

doc/Doxyfile

@@ -34,7 +34,7 @@ PROJECT_NAME           = Bitcoin
 # This could be handy for archiving the generated documentation or 
 # if some version control system is used.
 
-PROJECT_NUMBER         = 0.5.0
+PROJECT_NUMBER         = 0.9.0
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description 
 # for a project that appears at the top of each page and should give viewer 

src/bitcoind.cpp

@@ -13,6 +13,22 @@
 #include <boost/algorithm/string/predicate.hpp>
 #include <boost/filesystem.hpp>
 
+/* Introduction text for doxygen: */
+
+/*! \mainpage Developer documentation
+ *
+ * \section intro_sec Introduction
+ *
+ * This is the developer documentation of the reference client for an experimental new digital currency called Bitcoin (http://www.bitcoin.org/),
+ * which enables instant payments to anyone, anywhere in the world. Bitcoin uses peer-to-peer technology to operate
+ * with no central authority: managing transactions and issuing money are carried out collectively by the network.
+ *
+ * The software is a community-driven open source project, released under the MIT license.
+ *
+ * \section Navigation
+ * Use the buttons <code>Namespaces</code>, <code>Classes</code> or <code>Files</code> at the top of the page to start navigating the code.
+ */
+
 void DetectShutdownThread(boost::thread_group* threadGroup)
 {
     bool fShutdown = ShutdownRequested();