Added information about the new allowSpellCheck configuration option

docs/SlyEdit_ReadMe.txt

                          SlyEdit message editor
-                              Version 1.68
-                        Release date: 2019-08-09
+                              Version 1.71
+                        Release date: 2020-03-03
 
                                   by
 
@@ -327,6 +327,12 @@ allowEditQuoteLines               Whether or not to allow the user to edit
                                   false.  If this option is not specified, the
                                   default value is true.
 
+allowSpellCheck                   Whether or not to allow spell check.  Valid
+                                  values are true and false.  Defaults to true.
+                                  You might want to disable spell check if,
+                                  for instance, the spell check feature causes
+                                  an "out of memory" error on your system.
+
 useQuoteLineInitials              Whether or not to prefix quoted message lines
                                   with the previous author's initials when
                                   replying to a message.  Valued values are