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.

samples/rake-guard/Rakefile

@@ -0,0 +1,10 @@
+
+task default: [:characters, :skills]
+
+task :characters do
+  load 'src/characters.rb'
+end
+
+task :skills do
+  load 'src/skills.rb'
+end