Add missing link to #737 in README
Add missing link to #737 in README

I guess GitHub doesn't auto-link issues in README.md.

eXcomm/gratipay.com,studio666/gratipay.com,gratipay/gratipay.com,mccolgst/www.gittip.com,gratipay/gratipay.com,studio666/gratipay.com,studio666/gratipay.com,eXcomm/gratipay.com,studio666/gratipay.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,gratipay/gratipay.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,mccolgst/www.gittip.com,gratipay/gratipay.com