Remove alpha warning

Since we have done testing regarding arithmetic correctness in https://github.com/near/libbf-test, in the same test suite for the C++'s boost.multiprecision library. We consider the JavaScript runtime is reasonably secure for DApps and even financial DApps. The alpha warning is therefore removed. 

The contributing parts at beginning of the doc is duplicate to the `## Contributing` section later, so it's also removed

Author: ailisp

README.md

@@ -19,18 +19,6 @@
 
 A JavaScript library for writing NEAR smart contracts.
 
-> **Warning**
->
-> This SDK is currently in **[`Alpha`](https://github.com/near/near-sdk-js/releases/)**.
->
-> The JavaScript runtime has not been fully audited. For creating smart contracts that hold value please use [`near-sdk-rs`](https://github.com/near/near-sdk-rs).
->
-> Help contribute!
->
-> - [Report issues you encounter](https://github.com/near/near-sdk-js/issues) 🐞
-> - [Provide suggestions or feedback](https://github.com/near/near-sdk-js/discussions) 💡
-> - [Show us what you've built!](https://github.com/near/near-sdk-js/discussions/categories/show-and-tell) 💪
-
 ## Prerequisites
 
 - node >=14 <16.6.0 || >16.6.0