Mention only adding to context in README

This pull request adds the following to the README, under "Conventions":

---

### Good Practice

To allow rollbacks to work without fuss in organizers, interactors should only *add* to the context. They should not transform any values already in the context. For example, the following is a bad idea:

```ruby
class FindUser
  include Interactor

  def perform
    context[:user] = User.find(context[:user])
    # Now, context[:user] contains a User object.
    # Before, context[:user] held a user ID.
    # This is bad.
  end
end
```

If an organizer rolls back, any interactor before `FindUser` will now see a `User` object during the rollback when they were probably expecting a simple ID. This could cause problems.