Document Elasticsearch directory traversal attack and remote code execution

PR: 201834
Security: CVE-2015-5377
Security: fb3668df-32d7-11e5-a4a5-002590263bf5
Security: CVE-2015-5531
Security: ae8c09cb-32da-11e5-a4a5-002590263bf5
Approved by: feld (mentor)