Update README

[jenschelkopf]

Thanks for this awesome work 🎉

Here's a few suggestions for updating the README. The goals of this PR are:

• Reduce the emphasize on 'centrally managed.' In my conversations with GitHub customers, many expect 'centrally managed' to be templates. They expect developers to extend and customize these templates, and to be able to push down updates. To help reduce confusion between this and the future Centrally Managed Workflow Templates feature, I've made a few edits and reduced the emphasize on management.
• Add language to help a team determine whether this app is right for them, especially those with security concerns

[jenschelkopf]

@SvanBoxel Could you take another look at this, please?

[SvanBoxel]

Love it, thank you @jenschelkopf 🙇