Add description for rake tasks

You can see the descriptions by running `rake --tasks`.

docs/Rakefile

@@ -35,6 +35,7 @@ end
 
 Liquid::Template.register_filter(ExtraFilters)
 
+desc "Serve a live view of the website on http://localhost:5000/jq/"
 task :serve do
   begin
     Bonsai.log "Press Control+C to quit"
@@ -65,6 +66,7 @@ task :serve do
   end
 end
 
+desc "Build the website from the bonsai sources"
 task :build do
   Bonsai.root_dir = Dir.pwd
   Bonsai::Exporter.publish!
@@ -74,6 +76,7 @@ def load_manual
   YAML::load(File.open("content/3.manual/manual.yml"))
 end
 
+desc "Build the manpage from the bonsai source of the manual"
 task :manpage do
   Tempfile.open "manpage" do |f|
     manual = load_manual
@@ -103,6 +106,7 @@ task :manpage do
   end
 end
 
+desc "Collect jq unit test cases from the bonsai source of the manual"
 task :mantests do
   load_manual['sections'].each do |section|
     (section['entries'] || []).each do |entry|