Add npmjs version badge to README

Author: hunter547

README.md

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 # topstepx-api
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+[![NPM Version](https://badge.fury.io/js/leaflet.smoothgeodesic.svg?style=flat)](https://www.npmjs.com/package/topstepx-apo)
 A framework-agnostic TypeScript client for the TopstepX trading API with REST and SignalR data feeds via WebSocket.
 
 ## Features