Prior to this commit, the `Gemfile.lock` enforced the use of
Bundler v1.17.3 which was released in 2014.
On Ruby 3, this triggers the following deprecation warning:
```
/home/user/.rvm/gems/ruby-3.0.2@shoulda-matchers/gems/bundler-1.17.3/lib/bundler/shared_helpers.rb:118: warning: Pathname#untaint is deprecated and will be removed in Ruby 3.2.
```
Since the gem itself in v5.0.0 requires at least Ruby v2.6,
it seems fair to require a recent version of Bundler v2,
which in turn runs of every Ruby v2.3 or later.
Bundler 2.0 was released recently, but none of our lockfiles are
associated with this version — they all refer to Bundler 1.x.
Unfortunately, Travis is now attempting to use Bundler 2.0 by default,
so we have to ensure that it is using the correct version as well.
* Move section on documentation from README to CONTRIBUTING
* Add a setup script that contributors and maintainers can use to set up
a dev environment
[skip ci]
