v0.9.0

What's Changed

• Clarify new/existing GitHub App installation documentation by @joshuadavidthomas in #99
• add (a)get_or_create_from_event method to Installation manager by @joshuadavidthomas in #101
• 🔖 bump version 0.8.0 -> 0.9.0 by @joshuadavidthomas in #103

Full Changelog: v0.8.0...v0.9.0