[AIRFLOW-XXX] Add Poshmark to Companies (#4252)

* Add Poshmark to Companies

Author: Adityan

README.md

@@ -264,6 +264,7 @@ Currently **officially** using Airflow:
 1. [Plaid](https://www.plaid.com/) [[@plaid](https://github.com/plaid), [@AustinBGibbons](https://github.com/AustinBGibbons) & [@jeeyoungk](https://github.com/jeeyoungk)]
 1. [Playbuzz](https://www.playbuzz.com/) [[@clintonboys](https://github.com/clintonboys) & [@dbn](https://github.com/dbn)]
 1. [PMC](https://pmc.com/) [[@andrewm4894](https://github.com/andrewm4894)]
+1. [Poshmark](https://www.poshmark.com)
 1. [Postmates](http://www.postmates.com) [[@syeoryn](https://github.com/syeoryn)]
 1. [Pronto Tools](http://www.prontotools.io/) [[@zkan](https://github.com/zkan) & [@mesodiar](https://github.com/mesodiar)]
 1. [Publicis Pixelpark](https://www.publicispixelpark.de/) [[@feluelle](https://github.com/feluelle)]