Update DESIGN.md
Update DESIGN.md

Added implementation notes and performance tips
JackSullivan/spire,guersam/spire,adampingel/spire,kevinmeredith/spire,JackSullivan/spire,woparry/spire,codeaudit/spire,AlecZorab/spire,adampingel/spire,siddhartha-gadgil/spire,non/spire,rklaehn/spire,kevinmeredith/spire,siddhartha-gadgil/spire,codeaudit/spire,rklaehn/spire,guersam/spire,woparry/spire,tixxit/spire,tixxit/spire,non/spire,AlecZorab/spire