Add prelude-clean-whitespace-on-save to README.md

Also fix incorrect indentation in prelude-editor.el changes.
Good catch bbotsov!

README.md

@@ -312,6 +312,19 @@ personal config with the following bit of code:
 (setq prelude-whitespace nil)
 ```
 
+If you like `whitespace-mode` but prefer it to not automatically
+cleanup your file on save, you can disable that behavior by setting
+prelude-clean-whitespace-on-save to nil in your config file with:
+
+```lisp
+(setq prelude-clean-whitespace-on-save nil)
+```
+
+The prelude-clean-whitespace-on-save setting can also be set on a
+per-file or directory basis by using a file variable or a
+.dir-locals.el file.
+
+
 #### Disable flyspell-mode
 
 If you're not fond of spellchecking on the fly: