Add tip about merge over rebase

[mate-h]

After a quick discussion in discord, I decided to read through the contributing guidelines in Bevy and found there is no mention of rebase or merge, so I added this sentence!

[Zeophlite]

My two cents: when a PR is still drafting, I tend to rebase it a bit, I think maybe limit this to things that are S-Needs-Review or later (or similar)

[alice-i-cecile]

I agree with Francois's comment above :)