Improve the documentation about how to change the email templates

[vitormattos]

Now, the documentation says this:

To overwrite the default messages, create a new class inheriting from MessageProvider

But don't talk how to do this.

Ref:

This step is very strange to me because is a suggestion to do a change inside the app code. But now is the way that this app works. Is a point to think more about and suggest a best implementation.

Originally posted by @vitormattos in #30 (comment)