Add a Justfile and instructions to README

Justfile

@@ -0,0 +1,16 @@
+setup_and_test: setup test
+
+setup:
+  bin/setup
+
+test:
+  bundle exec rspec
+
+console:
+  bin/console
+
+install:
+  bundle exec rake install
+
+release:
+  bundle exec rake release

README.md

@@ -313,9 +313,13 @@ Here are some simple recommendations for the best results:
 
 ## Development
 
-After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake spec` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.
+After checking out the repo, use the [just](https://github.com/casey/just) command runner for common tasks:
 
-To install this gem onto your local machine, run `bundle exec rake install`. To release a new version, update the version number in `version.rb`, and then run `bundle exec rake release`, which will create a git tag for the version, push git commits and tags, and push the `.gem` file to [rubygems.org](https://rubygems.org).
+- `just setup`: Installs dev dependencies
+- `just test`: Runs the test suite
+- `just console`: Opens an IRb console with the gem loaded for experimenting.
+- `just install`: Install the gem on your local machine.
+- `just release`: Create and push a new release to the git repo and Rubygems. (Be sure to increase the version number in `version.rb` first!)
 
 ## Contributing