message: fix: update SDK readme to include warning
diff: The `@eth-optimism/sdk` package provides a set of tools for interacting with Optimistic Ethereum.
  +## NOTICE
  +
  +WARNING: This package is currently under construction.
  +You can definitely try to use it, but you probably won't have a good time.
  +Lots of pieces are missing.
  +We will announce the 1.0.0 release on Discord and on Twitter once it's ready, keep an eye out!
  +
  ## Installation
  ```
  