CONTRIBUTING.md - fix path for design principles doc (#975)

Update CONTRIBUTING.md

fix path for design principles doc

Author: Nathan Sullivan

CONTRIBUTING.md

@@ -15,7 +15,7 @@ There are [community meetings](https://calendar.google.com/calendar/u/0?cid=Y19m
 
 ## Design Principles
 
-When implementing a new feature please review the [design principles](./design/design-principles.md) to help guide the approach.
+When implementing a new feature please review the [design principles](./docs/design/design-principles.md) to help guide the approach.
 
 ## Development Environment
 
@@ -103,4 +103,4 @@ Commit messages should follow the general guidelines:
 - Messages should be descriptive:
   - First line, 50 chars or less, as a heading/title that people can find
   - Then a paragraph explaining things
-- Consider a footer with links to which bugs they fix etc, bearing in mind that Github does some of this magic already
+- Consider a footer with links to which bugs they fix etc, bearing in mind that Github does some of this magic already