Add flat_stream to experimental:

This adds a new stream wrapper class template designed to address
a performance shortcoming of boost::asio::ssl::stream.

doc/source.dox

@@ -105,6 +105,7 @@ WARN_LOGFILE           =
 INPUT = \
         $(LIB_DIR)/include/boost/beast/ \
         $(LIB_DIR)/include/boost/beast/core \
+        $(LIB_DIR)/include/boost/beast/experimental/core \
         $(LIB_DIR)/include/boost/beast/http \
         $(LIB_DIR)/include/boost/beast/websocket \
         $(LIB_DIR)/include/boost/beast/zlib