Change the next version number in the README

Updating the valid versions of acme means potentially causing conflicts
with other packages. We don't have any kind of compatibility guarantees
per se, but 0.2 seems to suggest this possibility more clearly than
0.1.2.

README.rst

@@ -95,7 +95,7 @@ Below is a summary of changes introduced in each release. Any user-visible
 changes *must* be recorded here. Note that the topmost entry typically
 represents the *next* (i.e. not yet released) version.
 
-0.1.2
+0.2.0
 +++++
 
 * Upgrade to acme 0.11.x