Sylvia/docs/anant calculator

@mhoeber @lamagnifica @catong @JAAkana This PR adds information about the edX calculator (including screen shots), per a request from Anant. Please let me know what you think. Thanks! 
pro tip: you *can* wrap a note! just put a hard carriage return at the appropriate point in the first line, and then indent the second line 1-2 spaces. When you do ctrl+alt+q on the 2nd line, the rest of the note text will wrap correctly.