auto-open Settings tab on extension install

[aspiers]

Use https://developer.chrome.com/docs/extensions/reference/api/runtime#event-onInstalled

Bard suggested:

chrome.runtime.onInstalled.addListener((details) => {
    if (details.reason === chrome.runtime.OnInstalledReason.INSTALL) {
        chrome.tabs.create({ url: chrome.extension.getURL("settings.html") });
    }
});

Once working, update Usage section of README too.