Add a guide for people migrating from airbnb, xo and standard

[alexewerlof]

if we are going to have differences, we need to facilitate transitioning from other popular configs and motivate the reason why we made a different choice.

[SimenB]

I don't think we need to justify changes from standard and xo, but we can point out the main differences. I agree on the airbnb case, though

And transitioning I'd think was mostly just eslint . --fix.

[alexewerlof]

I don't think we need to justify changes from standard and xo, but we can point out the main differences.

Deal 👍

And transitioning I'd think was mostly just eslint . --fix

Ideally all of our differences is something that --fix can handle.

[ivarconr]

close this then?

[alexewerlof]

It's WIP. Will close this when next is in good shape.