Add support for DESIGN.md

[fivenp]

Is your feature request related to a problem? Please describe.
Right now the solution with a design folder seems way too overkill. A lot of projects would benefit from something way leaner and easier to integrate...

Describe the solution you'd like
Let's also support DESIGN.md files in the root of the repo. It would be consistent to other standards like SECURITY.md or TESTING.md, etc