Mention in README that sticky notifications can be controlled by System Prefs

[KaranErry]

After playing around with sticky notifications, I realized that without using alerter I can get terminal-notifier notifications to stick by simply controlling it in System Prefs > Notifications preferences, like we do for all other macOS apps. A screenshot is attached of what I mean, i.e. macOS simply enforces all notifications issued by terminal-notifier to stick; it's not an app-level change.

This workaround should ideally be mentioned in the README, as an alternative to using alerter. Of course, this does not mean the notifications can show custom buttons, nor that the stickyness can be controlled on a per-notification basis; that is where alerter comes in. But for those using simple terminal-notifier notifications, this can enable them to simply be sticky with minimal extra effort on the user's end.

Instructions:

1. Open System Preferences > Notifications
2. Search for terminal-notifier in the list
3. Change "Banners" to "Alerts"

Please check everything that applies to your issue:

• I looked in closed issues and it has not already been answered
• My issue appeared with a terminal-notifier update
• I'm using a tool that uses terminal-notifier (pync: https://pypi.org/project/pync/)

To help us debug your issue please include:

• the macOS version you use: 11.6.2 (Big Sur)
• terminal-notifier version: 2.0.0
• how did you install terminal-notifier: Homebrew
• step-by-step reproduction instructions