Update DESIGN.md
Update DESIGN.md

Added something about SafeLong
adampingel/spire,codeaudit/spire,guersam/spire,woparry/spire,non/spire,guersam/spire,siddhartha-gadgil/spire,JackSullivan/spire,kevinmeredith/spire,AlecZorab/spire,tixxit/spire,adampingel/spire,rklaehn/spire,AlecZorab/spire,JackSullivan/spire,tixxit/spire,woparry/spire,siddhartha-gadgil/spire,kevinmeredith/spire,codeaudit/spire,non/spire,rklaehn/spire