add template Guardfile

Fix bspaulding/guard-rdoc#1
Let's leave the rdoc options blank in the template. I'd be down to suggest sdoc with some sort of comment:

```ruby
guard(:rdoc) do # use the rdoc default options
# or pass options to rdoc:
# guard(:rdoc, format: :sdoc, output: 'documentation') do
  watch(%r{.+\.rb$})
end
```