Post: mention flyout_display

Mention that people wanting to keep the flyout attached into the navbar,
has to define `flyout_display = "attached"`.

content/posts/addons-by-default.md

@@ -109,7 +109,7 @@ for example.
 
 Depending on your project's configuration, the documentation tool and the theme you are using, you will notice these differences.
 We released a new version of our theme to keep the flyout menu at the bottom left integrated with the navbar
-that you can install it as `sphinx-rtd-theme==2.1.0rc1`.
+that you can install it as `sphinx-rtd-theme==2.1.0rc2` and definining [`flyout_display`](https://sphinx-rtd-theme.readthedocs.io/en/latest/configuring.html#confval-flyout_display) as `attached`.
 We are also talking to other theme authors to communicate these changes and work with them towards a better integration.
 Expect news around this visual changes in the following months.