Fix DESIGN_GUIDELINES.md link in README.md.
Fix DESIGN_GUIDELINES.md link in README.md.

Taken from https://github.com/ChromeDevTools/devtools-frontend/pull/158

Tbr: 738c2fdc53f8b1553cee5c6a41ce003a6bb57f69@chromium.org
Change-Id: I37ed317cb5d42744df501877ee74d69ad152275d
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2113376
Reviewed-by: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>
Commit-Queue: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>

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