Documents workarounds for tmpfs space errors

tmpfs space errors result from creating a new seed image on
a system with less than 9GB of RAM. This patch documents
how to prevent the tmpfs space errors by avoiding the use of
tmpfs or specifying a minimum tmpfs size required.
Closes-Bug: 1215298

Change-Id: I89e63bb9a12988c0a4fa9fc781719070c98a0dfa
Merge "Documents workarounds for tmpfs space errors"