Migration guide to 0.15 not available

[drrnbrns]

I commented on #115 but perhaps it wasn't seen. The change log for 0.15 has this comment and link.

Upgrading from 0.14 requires code changes throughout: see the Forge2D migration guide

The main README.md has the same link. The link redirects to https://docs.flame-engine.org/latest/ which doesn't contain any such guide.

[spydon]

Yeah, it's waiting for this PR to be merged (we ran into some small problems), I thought we would have merged it faster.

[drrnbrns]

This can now be closed. The migration guide looks great!