Documentation housekeeping

This commit comprises minor documentation updates as flagged by Puppet
Strings. This includes:

- Moving the Japanese README to the root of the project (YARD does not
  know how to parse a readmes directory).
- Add a minimal description to the crontab provider.

Author: mhashizume

readmes/README_ja_JP.md renamed to README_ja_JP.md

@@ -2,6 +2,8 @@
 require 'puppet/provider/parsedfile'
 
 Puppet::Type.type(:cron).provide(:crontab, parent: Puppet::Provider::ParsedFile, default_target: ENV['USER'] || 'root', raise_prefetch_errors: true) do
+  desc 'The crontab provider'
+
   commands crontab: 'crontab'
 
   text_line :comment, match: %r{^\s*#}, post_parse: proc { |record|