Docs on using contentsmanagers

[jcrist]

Since the yarn containers are non-persistent, users will likely want to use a contentsmanager. A few reasonable options exist:

• https://github.com/jcrist/hdfscm (hdfs)
• https://github.com/quantopian/pgcontents (postgres)
• https://github.com/danielfrg/s3contents (s3 and gcs)

[jcrist]

Fixed by 0883366e1f54e156fad0b45d91d224ddac23402b