README.md: Intro and frequently used links
README.md: Intro and frequently used links

Change-Id: I9ff3bb8ae6f60e0eafa234e08c4ee0deaec94613
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/64040
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Anthony DiGirolamo <4cc40125edc24ec5bc72df7917ea0b403ee8cebe@google.com>

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