Update README.md

Please add MuPool.com

Why should you join MuPool.com to mine Dogecoin?

    Professional Software Developers
    Custom developed Queing-Cloud (https://mupool.com/queing.png) (Even if some of our servers are burning you get fully rewarded!)
    Fully virtualized Envoriment
    Proven Stability (0.5% of LTC-Network)
    Anti DDOS
    Low Fees 0.3%
    Multi Coin Support
    Block Finder Bonus for all Coins!
    PPLNS