Note requirements for exact class names

[Phrogz]

The example docs and code previously did not make it clear that the exact class names must be used for the add-on extension to function.

[CLAassistant]

All committers have signed the CLA.

[Phrogz]

These changes might seem a little pedantic; after all, the README for the exporter does already say "Add a class named …". However, the required class names looked to me like examples, and I spent 3.5 hours trying to figure out why my class (with a different name) wasn't being called, trying to find what I was missing to register my extension so it would be found. I never guessed convention over configuration would be in play. After all, the example files already have other classes like ExampleExtensionProperties.

So, I hope that these changes save some other developer some grief, by being abundantly clear.

[julienduroure]

Merged, thanks!