README: Update Elastic Search links to guide/en/...

The last commit in the old doc repository [1]:

  commit ee1a4774c671c0b4d087414806287e7786ea2107
  Author: Clinton Gormley <clint@traveljury.com>
  Date:   Sat Oct 5 17:25:12 2013 +0200

    Updated the README to point to the new docs location

The new README contents are:

  The Elasticsearch docs now live in the main repository:
  http://github.com/elasticsearch/elasticsearch

  See: https://github.com/elasticsearch/elasticsearch/tree/master/docs

I expect the new URLs are a result of this change.  One happy side
effect is that glossary entries are now identified with
'glossary-<entry>', so we no longer have a collision on the 'index'
ID.

[1]: https://github.com/elasticsearch/elasticsearch.github.com