Add warning section to README to tell users to upgrade to v1.0.1. (#264)

* Add warning section to README to tell users to upgrade to v1.0.1.

* Remove the DANGER warning label from README.

Author: CodeSchwert

README.md

@@ -12,6 +12,9 @@
 
 [![npm version](https://badge.fury.io/js/@imtbl%2Fcore-sdk.svg)](https://www.npmjs.com/package/@imtbl/core-sdk) [![Maintainability](https://api.codeclimate.com/v1/badges/219466ee5269620167e5/maintainability)](https://codeclimate.com/repos/62848fd8d4420d01b6002210/maintainability)
 
+> **⚠ We recommend you upgrade to Core SDK v1.0.1 as soon as you can.**  
+> We've released a critical bug fix in the stark `grindkey` method logic to work when there is a leading zero in the Ethereum wallet private key.
+
 The Immutable Core SDK provides convenient access to Immutable's APIs and smart contracts to help projects build better web3 games and marketplaces.
 
 Currently, our SDK supports interactions with our application-specific rollup based on StarkWare's StarkEx. In future, we'll be adding StarkNet support across our platform.