gn: Add a newline to fix a bulleted list in README.md.

Currently the bulleted list in the "What does 'done' mean?" section isn't
rendered (instead, it is shown as one paragraph with asterisks in the
middle). Adding a newline should fix this.

See https://chromium.googlesource.com/chromium/src/tools/gn/+/441de2f

Review URL: https://codereview.chromium.org/1153803002

Cr-Original-Commit-Position: refs/heads/master@{#331229}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 92acc5eca80b1cbd6fcf18a81795c5b74f9d3f06