Document when to join or not to join

[nathanl]

The README seems to suggest that joining your preloads is always more performant. That is not the case - see also this answer.

You might want to add a note suggesting when to join and when not to joni.

[joshnuss]

Hi nathanl,

Feel free to open a PR,

I haven't used this repo in a while, so if it's causing issues for people I'd also consider archiving it.