Add env var license instructions to readme

[carlin-q-scott]

The instructions here provide a better way to set the license than the example in the readme:

AutoMapper/docs/source/License-configuration.md

Lines 21 to 26 in dfa6dd5

	### Auto-Discovery via Environment Variables
	If no license key is set in code, AutoMapper looks for one in environment variables. This is convenient for containerized and cloud environments, and for enterprises that share a single key across many services without code changes:
	- `AUTOMAPPER_LICENSE_KEY` – the AutoMapper-specific license key.
	- `LUCKYPENNY_LICENSE_KEY` – a shared key usable across Lucky Penny products (for example, [MediatR](https://github.com/LuckyPennySoftware/MediatR) reads the same variable). Because it is shared, the key must be for a license that includes AutoMapper (a `Bundle` or AutoMapper edition); a MediatR-only license will not validate here.

[carlin-q-scott]

Your documentation site is also missing this better path for licensing.
https://docs.automapper.io/en/latest/License-configuration.html

[jbogard]

Not sure why the downvotes here lol. But there's something janky on the docs site, it's not picking up the latest version correctly. The docs are there on the "Stable" version.

[jbogard]

OK this is fixed, just a misconfigured in the docs site configuration.

[carlin-q-scott]

@jbogard Could you also consider adding it to the readme? I think a lot of people will see that first.

[jbogard]

Yeah, the readme would work too! I'll create a new issue for that.

[jbogard]

Actually, I'll just reopen this one since that was the original title.