fix(exporter-zipkin): add main property - entry point to the module (open-telemetry#289)

Author: mayurkale22

packages/opentelemetry-exporter-zipkin/package.json

@@ -2,6 +2,7 @@
   "name": "@opentelemetry/exporter-zipkin",
   "version": "0.0.1",
   "description": "OpenTelemetry Zipkin Exporter allows the user to send collected traces to Zipkin.",
+  "main": "build/src/index.js",
   "types": "build/src/index.d.ts",
   "repository": "open-telemetry/opentelemetry-js",
   "scripts": {