Add Steepfile to default Include list

[ybiquitous]

Steep 0.12.0 has introduced Steepfile.
See https://github.com/soutaro/steep/blob/v0.12.0/CHANGELOG.md#0120-2020-02-11.

We can write the Ruby DSL in Steepfile. For example:

target :app do
  check "lib"
  signature "sig"

  library "set", "pathname"
end

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[ybiquitous]

I'm not sure whether I should update CHANGELOG.md. 😓

[koic]

Yeah. Can you add a changelog entry?

[ybiquitous]

@koic Thanks for your reply. I've changed. Is it OK? 08f37d381c9d5ff60734cb826302157a17415014

[koic]

This looks good to me. Can you squash your commits into one?

[ybiquitous]

Yes! I've done it.

[koic]

Thanks!