[ci skip] add guide for ActiveModel::SecurePassword

As asked in the #15494, this is the PR for the `ActiveModel::SecurePassword` guide.
I'll move this last sentence out and merge it into the next paragraph, since they are both dealing with the setup/prerequisites/requirements.

Perhaps even with a separate header:

```
#### Requirements

In order to use `ActiveModel::SecurePassword`, you need to add `bcrypt` too your Gemfile. You'll also need to have...
```