DOC: Add note about RTD default rendered documentation version

When visiting docs.itk.org/, it will now redirect to the `release`
branch build, which is the documentation for the most recently tagged
version. `latest` will contain changes in `master` -- make a note on
this and how to find the rendering of `latest`.

Documentation/docs/README.md

@@ -9,6 +9,11 @@ These are the sources for the [ITK docs](docs.itk.org)
 The contribution process generally follows our [contributing
 guidelines](../../CONTRIBUTING.md).
 
+Once merged into `master`, the documentation will be available in the `latest`
+version of the ReadTheDocs documentation as https://docs.itk.org/. Note that
+this is not the default version. To change versions, use the links in the lower
+left of the page.
+
 To preview documentation changes, three options are available.
 
 ### Option 0: Edit on GitHub, use the pull request preview.