Make the document more clear and understandable

[huayuanzzb]

I saw the following statement from the document:

if the key is not present in the language file the message fallbacks to ${defaultContext}.ERRORS.${errorType} (USER.REGISTRATION.NAME.MINLENGTH => GENERAL.ERRORS.MINLENGTH)

However, when I tried to use it, I found that it does not always work as described in the document.

After I read the source code, I found that only after injecting my own MESSAGES_PROVIDER, it can work normally.

So, can you modify the document so that others can better use this exciting project?