Document how to limit candidate languages.

Closes #15

README.md

@@ -22,3 +22,15 @@ change it and stop automatic checks.
 You can use `adict-change-dictionary-hook` to hook into any of these changes,
 or `adict-conditional-insert` to insert text (like signatures) that will
 automatically conform to the language.
+
+To limit the candidate languages when guessing which dictionary to use, add
+something like the following to your Emacs config:
+```elisp
+(eval-after-load "auto-dictionary"
+ '(setq adict-dictionary-list
+ (mapcar 'adict--guess-dictionary-cons
+ '(("en" . "en")
+ ("fr" . "fr")))))
+```
+Get the full list from `adict-dictionary-list` in auto-dictionary.el, or
+alternatively use the Customize interface to do this.