Update readme message about v2 support

[trivikr]

Suggests the following changes to the message added in #4281

• Breaks the message into three paragraphs for improved readibility:
  • Announcement
  • Get help with migration
  • Subscribe to updates and announcements
• Uses topic for v3-recommended-blog instead of "this blog"
• Adds explicit link to aws-sdk-js-codemod for folks who may not read the blog.
• Uses language with "you" as an actor for the messaging about experimental scripts.

Rendered version can be viewed at https://github.com/trivikr/aws-sdk-js/blob/v3-recommended-blog-topic/README.md

Checklist

• non-code related change (markdown/git settings etc)