docs: begin writeup of guard

While writing this I realized that this really should be part of part_3_workflows.rst, so it'll eventually make it into there. But it's recorded in its unpolished form for now.

spec/samples/run_samples_spec.rb

@@ -11,7 +11,7 @@ describe 'Squib samples' do
       Dir.chdir(File.dirname(sample)) do
         load sample
       end
-    end
+    end unless sample =~ /rake-guard/ # ignore our project sample, run it below
   end
 
 end