Update README for v2

[fabianfett]

Expected behavior

N/A

Actual behavior

N/A

Steps to reproduce

N/A

If possible, minimal yet complete reproducer code (or URL to code)

No response

What version of this project (swift-aws-lambda-runtime) are you using?

N/A

Swift version

N/A

Amazon Linux 2 docker image version

No response

[sebsto]

Link to docc hosted on Swift Package Index

[sebsto]

In progress with
#381

[sebsto]

Missing a paragraph about Swift Service Lifecycle
#374

[sebsto]

Adding a note about current status of Swift Service Lifecycle
#461

I consider the README v2 to be complete. Any future modification can be part o a new issue / PR. Closing this.