Doc: Added information about the documentation in qt5/README file.

-added basic information and links to wiki and qtbase/doc/README file.

Task-number: QTBUG-29057
Change-Id: I21d4dbaf7c1d03b63949af1fa303cb6fcbee8949
Reviewed-by: Johanna Äijälä <johanna.aijala@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>

README

@@ -122,3 +122,22 @@ HOW TO BUILD QT5
  =====================
  See http://qt-project.org/wiki/Building-Qt-5-from-Git and README.git
  for more information.
+
+
+ Documentation
+ =============
+
+ After the configure step, building the documentation is possible by running
+ "make docs".
+ Example:
+
+    ./configure -prefix $PWD/qtbase -opensource
+    make docs
+
+ The documentation is installed in the path set to $QT_INSTALL_DOCS.
+ Running "qmake -query" will list the value of QT_INSTALL_DOCS.
+
+ Information about Qt 5's documentation is located in qtbase/doc/README
+ or in the following page: http://qt-project.org/wiki/Qt5DocumentationProject
+
+ Note: Building the documentation is only tested on desktop platforms.