Make the getting started section in the README better
Make the getting started section in the README better

Some of my colleagues have commented in the past few months that the
Breakpad README is not very clear on how to get and build the code
nowadays. This change moves some of the docs from the "request
change review" section up to the "getting started" section, and adds
a few more things to clarify.

R=vapier@chromium.org
BUG=

Review URL: https://codereview.chromium.org/2035103002 .

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