elementary provides badges for your GitHub README. Badges will automatically open AppCenter or redirect to elementary.io based on OS detection. They look like this:
[](https://appcenter.elementary.io/com.github.USER.REPO)
<a href="https://appcenter.elementary.io/com.github.USER.REPO"><img src="https://appcenter.elementary.io/badge.svg" alt="Get it on AppCenter"></a>
Make sure to replace com.github.USER.REPO
in the URL with the ID for your app