Exclude app/assets/**/* by default

[r7kamura]

This change is for the same reason as the following issue:

• Ignore more directories #796

In a typical Rails application, this directory often contains a large number of non-ruby files, and I believe this optimization is reasonable for rubocop-rails to speed things up.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Can you rename this filename to changelog/change_exclude_app_assets_by_default.md?

[koic]

Thanks!