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..

This includes the following classes:

- ActiveModel::Serializers::Xml
- ActiveRecord::Serialization::XmlSerializer

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? 😁

ActiveModel::Errors

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

The four code samples that fail to run are:

- Add attribute magic to objects. Fixed by introducing a Person
  instance variable.

- Tracking value changes. Fixed by replacing `attr_accessor` with
  `define_attribute_methods`, providing getter and setter methods
  for `name` and providing the missing `Person#save` method. A
  call to `Person#save` has to precede the `person.name = 'robert'`
  assignment, if we want `previous_changes` to include 'bob'.

- Adding `errors` interface to objects. Fixed by introducing a
  Person instance variable, assigning `nil` to its name and calling
  `Person#validate!`.

- Custom validators. Fixed by defining `HasNameValidator` before
  it is used by `ValidatorPerson`.

All the code samples can now be run smoothly.

Call Dirty#changes_applied in Person#save, instead of modifying instance vars.

This reverts commit c962680e.

Reason: The links must be relative to work in both edge and stable api
sites.

This reverts commit ddb98755.

Reason: The links must be relative so that they will work both in edge
and stable api sites.

Also add test to ensure basic model does not explode when
initialized with nil.

This reverts commit d163d3bf.

Reason: The alternate approach can be documented in the guide and
not disrupt the existing flow in the readme.

This reverts commit 36a5f48f.

Reason: Editing .gitignore in docrails is not allowed. Docrails is meant
only for documentation.