Add a note highlighting MongoDB disk requirements

Added a note to highlight the disk requirements of MongoDB
when database journaling is enabled (default). Provided
instructions for switching to smaller files and a link
to instructions for disabling journaling entirely.

Change-Id: Ie7410018ce664cddd66309df8d1b1bb17abdfcbb
Closes-Bug: #1253214
Merge "Add a note highlighting MongoDB disk requirements"