docs: add warning about machineid

[mathnogueira]

We had problems when running the keygen example on Docker and after some debugging we found out the machineid file was missing and it was making the license verification to fail.

As most applications will run on Docker, a warning about this would be useful.

[ezekg]

I don't think adding this warning is necessary, simply because it's not an official dependency of this SDK. Instead, I'd rather find and link to a fork of the machineid package that supports Docker, because it looks like that package is unmaintained. See denisbrodbeck/machineid#5.

For example, we maintain a Python library that is based on that Go package and the community added Docker support using the container ID: https://github.com/keygen-sh/py-machineid.

Perhaps we can fork and do the same for that unmaintained Go package. Feel free to make a PR: https://github.com/keygen-sh/machineid

[ezekg]

Alternatively, we could update the example to use https://github.com/panta/machineid, which is a fork that loosely supports Docker by writing a UUID to one of the ID files if no ID exists.

[mathnogueira]

I decided to open a PR to your fork instead. Panta's fork seems to be unmaintained as well because there's a PR there with an important fix that is open for more than an year.