Add local rake task for regenerating the example code

Rather than having to try and remember or look up the command to run code generation for the example, hit the easy button and create a simple rake task for it.

tasks/twirp_protoc_plugin.rake

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require "rake"
+
+desc "Re-runs code generation for the example using local plugin code."
+task :example do
+  %x(protoc --plugin=protoc-gen-twirp_ruby=./exe/protoc-gen-twirp_ruby \
+      --ruby_out=. \
+      --twirp_ruby_out=. \
+      ./example/hello_world.proto
+  )
+end