Document how to do self-templating

[yajo]

Self-templating templates suffer from a 🐔 and 🥚 problem that disallows them to use a valid copier-answers file. A workaround is documented in copier-org/autopretty#2, but it would be better to improve this.

There seems to be some bug regarding how files/dirs ended with .tmpl override the ones without that extension.

Examples:

• If my answers file is in .copier-answers.autopretty.upstream.yml and *.upstream.* is in _exclude, then it shouldn't be copied. But since it's my answers file, I need it updated. What to do?

[yajo]

Finally managed to do it. It was as simple as using _subdirectory 🤷🏼‍♂️

See https://github.com/copier-org/autopretty/tree/65aeb34041cbe2281c57be0c04635ed2c61c4c80#notes-for-template-maintainers

Gotta document this.

[yajo]

Fixed in df2a301.