Try to improve README for new users

[eaddingtonwhite]

When trying our go SDK for first time new users reported they had trouble following how to set up and install a fresh new empty go project with current install instructions.

• We should look at more SDK's in community and see if better way to surface how to bootstrap a fresh project with go.mod and installing our SDK's
• We should look into adding a section somewhere about IDE setup for go and see if any recomendations for go or vscode to make the dev experience better

[joseph-wortmann]

Especially the "AuthClient". Use of this for key rotation is unclear. Once Issue #571 is resolved (either through a fix or through reduction in my ignorance) I'm happy to contribute a Go key rotator for AWS.