Add Mercari to the company list

Closes apache#2424 from yu-iskw/add-mercari

README.md

@@ -140,6 +140,7 @@ Currently **officially** using Airflow:
 1. [Madrone](http://madroneco.com/) [[@mbreining](https://github.com/mbreining) & [@scotthb](https://github.com/scotthb)]
 1. [Markovian](https://markovian.com/) [[@al-xv](https://github.com/al-xv), [@skogsbaeck](https://github.com/skogsbaeck), [@waltherg](https://github.com/waltherg)]
 1. [Mercadoni](https://www.mercadoni.com.co) [[@demorenoc](https://github.com/demorenoc)]
+1. [Mercari](http://www.mercari.com/) [[@yu-iskw](https://github.com/yu-iskw)]
 1. [MiNODES](https://www.minodes.com) [[@dice89](https://github.com/dice89), [@diazcelsa](https://github.com/diazcelsa)]
 1. [MFG Labs](https://github.com/MfgLabs)
 1. [mytaxi](https://mytaxi.com) [[@mytaxi](https://github.com/mytaxi)]