Update README.md

Author: eregon

README.md

@@ -170,6 +170,7 @@ This action might work with [self-hosted runners](https://docs.github.com/en/act
 if the [virtual environment](https://github.com/actions/virtual-environments) is very similar to the ones used by GitHub runners. Notably:
 
 * Make sure to use the same operating system and version.
+* Set the environment variable `ImageOS` to the corresponding value on GitHub-hosted runners (e.g. `ubuntu18`/`macos1015`/`win19`). This is necessary to detect the operating system and version.
 * Make sure to use the same version of libssl.
 * Make sure that the operating system has `libyaml-0` installed
 * The default tool cache directory (`/opt/hostedtoolcache` on Linux, `/Users/runner/hostedtoolcache` on macOS,