freeze: rename README.rst to README.txt

The only reason I had a "rst" extension was so that GitHub would pick it
up.

--HG--
rename : Demos/freeze/README.rst => Demos/freeze/README.txt

bin/cython_freeze

@@ -3,7 +3,7 @@
 Create a C file for embedding one or more Cython source files.
 Requires Cython 0.11.2 (or perhaps newer).
 
-See Demos/freeze/README.rst for more details.
+See Demos/freeze/README.txt for more details.
 """
 
 import optparse