Add the Waffle.io badge to our README
Add the Waffle.io badge to our README

Signed-off-by: Robb Kidd <a629c362624370ec351e141df0bd1d27621ff790@thekidds.org>

rubyforgood/habitat_humanity,rubyforgood/habitat_humanity,bjmllr/habitat_humanity,rubyforgood/habitat_humanity,bjmllr/habitat_humanity,bjmllr/habitat_humanity