Add images and section details

[HidemichiShimura]

Description

• Add images such as gif and png to make it easier for viewers to understand what this app can do
• Add more details to "Installation on a local environment" and "Usage"

Implementation

• Embed a gif file of Slack app demo at beginning
• Embed Node icon png in "Built with"
• Embed a screenshot of Request URL in "Usage"
• Add scopes and event settings to "Slack App setup"
• Add CLI commands

[HidemichiShimura]

Thank you for your review!

I'll give you more comments once it is merged

I understood, thank you!