Provide description of requirements for running Unity CI actions on a self-hosted runner

**Is your feature request related to a problem? Please describe.**
Currently afaik Windows and other OSs are unsupported so I can't use a self-hosted runner without knowing about the capabilities expected by this action (or I need to install Ubuntu beside Windows on my system which may be undesirable unless the GitHub self-hosted runner application works through WSL, which is harder to set up than on a Linux distro which [supports `systemd`](https://docs.github.com/en/free-pro-team@latest/actions/hosting-your-own-runners/configuring-the-self-hosted-runner-application-as-a-service)).

**Describe the solution you'd like**
A description page which provides clear information on what configuration is expected from a machine which shall run this action.

**Describe alternatives you've considered**
The only other alternative is to go into the repo and dive into the code to find what capabilities are expected.

**Additional context**
When I first tried running the Activation action using `windows-latest` I was greeted with an error which read something like `win32 is not supported`.