gn: Add a newline to fix a bulleted list in README.md.
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-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#331229}

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