Currently there is no Adaptive Form OOTB component, capable to take input as 'rich text'.
You can write a custom component using custom widget, which supports rich text input capabilities. Following are the links for how to write a custom widget:
 https://git.corp.adobe.com/pages/vdua/sample-xfa-widget-tutorial/jekyll/update/2014/04/09/ creating-a-mobile-form-widget.html
Static Component looks like appears on the form as a constant text or basically for supplying additional information for users of the form.
What I am looking for is something like:
Additional Description ........(RTE)
The above "Additional Description" field should be sent on form "submit" in addition to the other fields of the form.
Please use 'Static text' component available in side-kick under 'Adaptive Form'. It has rich text capabilities.
Its already there. Have a look at test component of adaptive form.