Update //mojo/README.md to point to the new documentation.
Update //mojo/README.md to point to the new documentation.

BUG=577693

Review-Url: https://codereview.chromium.org/1926603002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390092}

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