DOC: small tweaks on the ddp3 extension in README.

Author: dragonyanglong

README.rst

@@ -48,10 +48,10 @@ installed using ::
 To install the remaining packages see the installation instructions
 at their respective web pages.
 
-The Python3 PDFgui project filename extension has been changed to .ddp3.
-Please note that sometimes it may fail for .ddp project files that were
-saved by the Python2 PDFgui to be read by Python3 PDFgui. We are working
-on a solution that will be available in a future version.
+Please note that the Python3 PDFgui will read .ddp3 files. It is also
+possible for it to read .ddp files that were saved by the Python2 PDFgui
+but it will sometimes fail to read these. We are working on a solution
+that will be available in a future version.
 
 INSTALLATION
 ------------------------------------------------------------------------