Specify Node version for Wasm BigInt support

From Node v15.0, Wasm BigInt support is no longer experimental

See: nodejs/node@ca8f3ef2e5

Author: dunxen

README.md

@@ -60,7 +60,7 @@ Some functions may error spuriously due to oversights or missing implementations
 
 The TypeScript bindings require modern web standards, including support for `FinalizationRegistry`
 and `WeakRef` (Chrome 84, Firefox 79, Safari 14.1/iOS 14.5 and Node 14.6) and WASM BigInt support
-(Chrome 85, Firefox 78, Safari 14.1/iOS 14.5, and Node ??).
+(Chrome 85, Firefox 78, Safari 14.1/iOS 14.5, and Node 15.0).
 
 ## General