Document code migration steps required for backwards incompatible changes.

### Suggestion: Create a migration document

It would be really helpful if backward incompatible changes were added to a "migration"/"upgrade" document.

This is a really good example of such an approach:

* https://github.com/embedded-graphics/embedded-graphics/blob/master/MIGRATING-0.5-0.6.md#upgrading-from-the-old-rgb565-color-type

(Although probably more comprehensive than what would be needed for `godot-cpp`.)

### Example of recent changes required to build existing code

For the record, I needed to do the following to get [a project to build that had built with an earlier version](https://github.com/follower/foreigner/tree/e10193990f8b02a8ee92a35c23b87768d678d18a) to build with the current version (77d41fa179e40560f1e264ed483638bf51713779):

* Change all references to `godot_headers` to be `godot-headers` instead. (Due to #519?)

* Remove all "manually" defined implementations of `___get_class_name()`, e.g.: (Due to #454?)

```C++
static inline const char *___get_class_name() {
return (const char *) "Foreigner";
}
```