feat!: Implement blank_gem that does nothing.

README.md

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+# BlankGem
+
+A blank/empty Ruby gem that does nothing. Useful for testing that only specific
+Bundler groups are loaded when using a dummy group.
+
+## Installation
+
+Add this line to your application's Gemfile, ideally under a group you'll never
+want:
+
+```ruby
+gem 'blank_gem', group: :never
+```
+
+And then execute:
+
+    $ bundle install
+
+## Usage
+
+Simply set Bundler's `without` option to exclude the group you've put the
+`blank_gem` within. For example:
+
+    $ bundle config set without 'never'
+
+## Contributing
+
+Bug reports and pull requests are welcome on GitHub at
+https://github.com/jimeh/blank_gem.