Add a job consideration doc

Discuss about the dual-engine usage consideration and
add suggestions around how this can be alleviated when
a jobboard is used to claim & perform work.

(Removed from gate, zuul lost this change and it log
jammed the gate.)

Change-Id: I0c2460fc3ff7e9d34b7578d2e967f6bfa489e770
Merge "Add a job consideration doc"