Add citation to design documentation for newbies
Add citation to design documentation for newbies

Add a citation in doc/design.md to an introduction to basic
web application concepts, in case the reader isn't familiar
with them.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

coreinfrastructure/best-practices-badge,linuxfoundation/cii-best-practices-badge,yannickmoy/cii-best-practices-badge,yannickmoy/cii-best-practices-badge,linuxfoundation/cii-best-practices-badge,coreinfrastructure/best-practices-badge,linuxfoundation/cii-best-practices-badge,coreinfrastructure/best-practices-badge,yannickmoy/cii-best-practices-badge,linuxfoundation/cii-best-practices-badge,yannickmoy/cii-best-practices-badge,coreinfrastructure/best-practices-badge