Add rakefile

Rakefile

+Dir.glob('lib/tasks/*.rake').each {|r| import r}

just-the-docs.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/pmarsceill/just-the-docs"
   spec.license       = "MIT"
 
-  spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r{^(assets|bin|lib|_layouts|_includes|_sass|LICENSE|README)}i) }
+  spec.files         = `git ls-files -z`.split("\x0").select { |f| f.match(%r{^(assets|bin|lib|_layouts|_includes|_sass|LICENSE|README|Rakefile)}i) }
 
   spec.add_runtime_dependency "jekyll", "~> 3.3"