Document how to use image pull policy for faas-netes

[milsonian]

This still needs to be done, opening so I don't lose track of it.

[alexellis]

Thanks @milsonian - this is 2 months old now, please can you give an ETA on when you can work on it?

[alexellis]

@LucasRoesler maybe it would be best if you took over this task which appears to have been dropped?

[LucasRoesler]

Sure, is this just updating the docs or a readme? Not a lot of details here

[alexellis]

Docs I think 👍