add note to import sources

lib/gitlab/import_sources.rb

@@ -7,6 +7,7 @@ module Gitlab
   module ImportSources
     ImportSource = Struct.new(:name, :title, :importer)
 
+    # We exclude `bare_repository` here as it has no import class associated
     ImportTable = [
       ImportSource.new('github',         'GitHub',        Gitlab::GithubImport::ParallelImporter),
       ImportSource.new('bitbucket',      'Bitbucket',     Gitlab::BitbucketImport::Importer),