Support for Deepl's free API account

[timnilson]

I think it's worth pointing out in the documentation that i18n-tasks works with the free API from Deepl. Not just the Pro account, which costs a monthly fee and a usage fee. The free account is limited to 500,000 characters per month, I believe.

Use it with these settings in the yml file:

translation:
deepl_api_key: "ba140f3d-xxx"
deepl_host: "https://api-free.deepl.com"
deepl_version: "v2"

[martijn10kb]

Yes, very much worth pointing out!

[davidwessman]

Could you please make a PR adding it to the README? @timnilson @martijn10kb