doc: add ThreadSafeFunction to main README (#513)

PR-URL: #513
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: NickNaso <nicoladelgobbo@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

README.md

@@ -106,6 +106,7 @@ The following is the documentation for node-addon-api.
  - [Async Operations](doc/async_operations.md)
     - [AsyncWorker](doc/async_worker.md)
     - [AsyncContext](doc/async_context.md)
+ - [Thread-safe Functions](doc/threadsafe_function.md)
  - [Promises](doc/promises.md)
  - [Version management](doc/version_management.md)