Update doc, write about add_test_dependency

doc/guides/mrbgems.md

@@ -153,6 +153,9 @@ MRuby::Gem::Specification.new('c_and_ruby_extension_example') do |spec|
 
   # Use latest mruby-onig-regexp from github. (version requirements can be omitted)
   spec.add_dependency('mruby-onig-regexp', :github => 'mattn/mruby-onig-regexp')
+
+  # You can add extra mgems active only on test
+  spec.add_test_dependency('mruby-process', :github => 'iij/mruby-process')
 end
 ```