To notify deprecation and make the migration easier.
Such warnings are added to a linting result and reporting output.

For example of a report:

```console
$ cd scripts
$ ../bin/stylelint.js visual.css --config=visual-config.json

Deprecation Warning: The "string-quotes" rule is deprecated and will be removed in the next major release.
Deprecation Warning: The "unit-case" rule is deprecated and will be removed in the next major release.

visual.css
 1:9   ✖  Expected "url("foo.css")" to be ""foo.css""       import-notation
 1:13  ✖  Expected single quotes                            string-quotes
 4:9   ✖  Unexpected invalid hex color "#4f"                color-no-invalid-hex
 6:1   ✖  Expected ".foo.bar" to have no more than 1 class  selector-max-class
 7:10  ✖  Unexpected unit "px"                              unit-disallowed-list
 8:3   ⚠  Expected custom property name to be kebab-case    custom-property-pattern

6 problems (5 errors, 1 warning)
```