Rewrite rubygems guide from HAML to Markdown

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+## Using Bundler while developing a gem
+
+If you're creating a gem from scratch, you can use bundler's built in gem skeleton to create a base gem for you to edit.
+
+~~~
+$ bundle gem my_gem
+~~~
+
+This will create a new directory named `my_gem` with your new gem skeleton.
+If you already have a gem, you can create a Gemfile and use Bundler to manage your development dependencies. Here's an example.
+
+~~~ruby
+source "https://rubygems.org"
+gemspec
+gem "rspec", "~> 3.9"
+gem "rubocop", "0.79.0"
+~~~
+
+In this Gemfile, the `gemspec` method imports gems listed with `add_runtime_dependency` in the `my_gem.gemspec` file, and it also installs rspec and rubocop to test and develop the gem.
+All dependencies from the gemspec and Gemfile will be installed by `bundle install`, but rspec and rubocop will not be included by `gem install mygem` or `bundle add mygem`.
+Runtime dependencies in your gemspec are treated as if they are listed in your Gemfile, and development dependencies are added by default to the group, `:development`.
+You can change that group with the `:development_group` option
+
+~~~ruby
+gemspec :development_group => :dev
+~~~
+
+As well, you can point to a specific gemspec using `:path`. If your gemspec is in `/gemspec/path`, use
+
+~~~ruby
+gemspec :path => '/gemspec/path'
+~~~
+
+If you have multiple gemspecs in the same directory, specify which one you'd like to reference using `:name`
+
+~~~ruby
+gemspec :name => 'my_awesome_gem'
+~~~
+
+This will use `my_awesome_gem.gemspec`
+That's it! Use bundler when developing your gem, and otherwise, use gemspecs normally!
+
+~~~
+$ gem build my_gem.gemspec
+~~~