Add lore about using non-standard merge options in GitHub

[mlangbehn]

Per @j2sol:

I think we should not use any merge strategy that would change the sha of the commit being merged in, as that will likely throw zuul off if github changes the value of A on merge, that'll invalidate tests of B on A and C on B on A

i.e. Zuul is only expected to behave as expected when using: Settings > Merge button > Allow merge commits