Document that cmswidget needs to be in extraPlugins setting

[stefanw]

For my installation, I had customised the extraPlugins setting and did not have "cmswidget" in there (and it used to worked).

With the latest version 3.1.0, when I opened a Text plugin, this line in plugins.js throws an error:

// Uncaught TypeError: Cannot set property 'inline' of undefined
that.editor.widgets.registered.cmswidget.inline = true;

that.editor.widgets.registered does not contain cmswidget.

I had to add 'cmswidget' in the list of extraPlugins to make this work. This doesn't seem to be documented. Having a list of required "extraPlugins" for djangocms-text-ckeditor to work might be a good idea.

[czpython]

Fixed by #355

[vxsx]

Will be properly fixed by #361