According to pr #22443 in the guides there's always a dollar sign before every command, so why is in the main README a `$` and in every submodule a `%`?

Just eye candy..

I think we are better off leaving `sudo` outside of the documented
way of installing gems (`activerecord`, `actionpack`, …).

We don’t want newbies to think that `sudo` is required or, even worse, than
they actually have to type `[sudo] gem install`.

In most scenarios, `sudo` is not needed to install gems, and people who do
need it, probably already know about it.

What do you think? 😁

- Wrapped text in setting defaults section. Also added break, to a sentence.
[ci skip]

the issue tracker. See also #15455 [ci skip]

This commit also addresses rails/docrails#169 and rails/rails#14159

Highlighted code

At 4a6eba32 mail sending example was rewrited from rails 2 format to
modern rails 3 approach. But passing parameter `recipient` to method
Notifier#welcome was lost.

Passing email as parameter makes this example more solid.

Added link in MIT license for Action Mailer README. I think this seems appropriate but commentary on this is not a bad idea.

Made the defaults section a little more readable and more to the point, giving a overview of the possibilities.