add cross-dev Gemfile and update 'bootstrap_form' gem

* added cross-dev Gemfile
* fixed some typos
* updated dev wiki link in README.md
* updated 'bootstrap_form' gem in Gemfile

the 'sigmike/bootstrap_form' gem added to the Gemfile in PR #74 has been merged upstream (https://github.com/bootstrap-ruby/rails-bootstrap-forms/pull/76)
  it was added apparently as a bugfix but is now quite outdated - it was upgraded in the peer4commit repo in june https://github.com/sigmike/peer4commit/commit/608980f7bd7b9fde5366180142f050d7baaf27a1