Add protip about disabling ExecutePermissions

Some people have been confused about how to use this hook when they have
files that should have execute permissions (sds#339). I am hoping that this
comment will help out future people.

Author: lencioni

lib/overcommit/hook/pre_commit/execute_permissions.rb

@@ -2,6 +2,16 @@ module Overcommit::Hook::PreCommit
   # Checks for files with execute permissions, which are usually not necessary
   # in source code files (and are typically caused by a misconfigured editor
   # assigning incorrect default permissions).
+  #
+  # Protip: if you have some files that you want to allow execute permissions
+  # on, you can disable this hook for those files by using the `exclude` option
+  # on your .overcommit.yml file. Example:
+  #
+  #   ExecutePermissions:
+  #     enabled: true
+  #     exclude:
+  #       - 'path/to/my/file/that/should/have/execute/permissions.sh'
+  #       - 'directory/that/should/have/execute/permissions/**/*'
   class ExecutePermissions < Base
     def run
       file_modes = {}