Add rdoc comments to classes.

lib/format-staged/entry.rb

@@ -1,6 +1,10 @@
 # frozen_string_literal: true
 
 class FormatStaged
+  ##
+  # Entry in the git index.
+  #
+  # Data as produced by `git diff-index`
   class Entry
     PATTERN = /^
       :(?<src_mode>\d+)\s

lib/format_staged.rb

@@ -6,6 +6,8 @@ require 'format-staged/entry'
 require 'format-staged/io'
 require 'shellwords'
 
+##
+# Runs staged changes through a formatting tool
 class FormatStaged
   attr_reader :formatter, :patterns, :update, :write, :verbose