Issue I encountered with install

I was trying to install guard today and ran into an issue:

mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/include/ruby.h

I found this post - castwide/vscode-solargraph#78 - which suggested the problem is because my mac is 10.14 

I had to run this command which then allowed me to install guard.

open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

I wanted to mention this to your team incase you want to include it, though you may not feel it is relevant to Ada students!

David

02-intermediate-ruby/testing-with-guard.md

@@ -19,6 +19,8 @@ Before we can use Guard to automatically run our tests while we work on projects
 $ gem install guard guard-minitest
 ```
 
+-> Issue with installing guard - see commit description
+
 After we've got Guard installed, we can start it by running the `guard` command in a terminal. Leave this terminal window open while you work, and Guard will display the results of your tests whenever they are re-run automatically.
 
 _Important_: You need to run `guard` from the "root" directory for the project (the directory which has the `Guardfile`).