Add top-level namespace docs

src/spectator/formatters.cr

@@ -1,6 +1,7 @@
 require "./formatters/*"
 
 module Spectator
+  # Reports test results to the end-user in various formats.
   module Formatters
   end
 end

src/spectator/matchers.cr

@@ -1,6 +1,8 @@
 require "./matchers/*"
 
 module Spectator
+  # Evaluates conditions on values and objects
+  # to determine whether they satisify test criteria.
   module Matchers
   end
 end