Similar to #470, only this should be the transition guide from 1.0 to 2.0.

I think the main reason why #470 failed to materialize was that the starting point was too vague, and that the amount of work that was needed to implement such guide was too large.

We can, however, take a different approach this time: create a special MIGRATION.md document, and require that every breaking feature was listed there together with a short explanation on how to migrate. That way the guide would be built incrementally in small steps.

"Implementing" this issue requires:

• creating a blank document MIGRATION.md (or however we want to call it);
• updating the contributing guidelines;
• updating the PR template to mention the new document.