doc: update readme for removal of documentation

doc/README.md

@@ -1,19 +1,14 @@
 # Documentation
 
-This `doc` folder is the future location of documentation for Subiquity.   This
-will be used to migrate the official documentation to readthedocs, and later
-will be the home of that documentation.
+This `doc` folder is the location of documentation for Subiquity.
 
-The `../documentation` folder is the existing location for this sort of
-documentation.  As the RTD version is not yet ready, documentation changes
-should be made to `../documentation` until this README is updated to say
-otherwise.  As such, please make any desired documentation changes to
-`../documentation`.
+In the not distant past, there was also a `../documentation` folder, but that
+is now an obsolete copy of what was on Discourse.
 
 # Local preview
 
 To build this documentation, you can run `make install` from this folder to
-create the virtual environment. 
+create the virtual environment.
 
 Then run the `make run` command, which will build a html version of the docs,
 and serve the docs in the virtual environment. This is very convenient if you