bundle init
Generates a Gemfile into the current working directory
$ bundle init [--gemspec=FILE]
Options:
--gemspec: Use the specified .gemspec to create the Gemfile
Init generates a default Gemfile in the current working directory. When adding a
Gemfile to a gem with a gemspec, the --gemspec option will automatically add each
dependency listed in the gemspec file to the newly created Gemfile.