Add flyspell (#549)

Author: jcs090218

README.org

@@ -913,6 +913,7 @@ External Guides:
 
 ** Note
 
+    - [[https://www.gnu.org/software/emacs/manual/html_node/emacs/Spelling.html][flyspell]] - =[built-in]= On-the-fly spell checker.
     - [[http://www.gnu.org/software/emacs-muse/][Emacs Muse]] - a publishing environment for Emacs.
     - [[https://github.com/rnkn/fountain-mode/][Fountain Mode]] - a full-featured screenwriting environment for GNU Emacs using the Fountain markup format.
     - [[https://github.com/joostkremers/writeroom-mode][writeroom-mode]] - Writeroom-mode: distraction-free writing for Emacs.