Move CONTRIBUTING.md to rustc-dev-guide#753

Merged

mark-i-m merged 7 commits intorust-lang:masterfrom

mark-i-m:contributing

Jul 2, 2020

Commits on Jul 2, 2020

move CONTRIBUTING.md to rustc-dev-guide
mark-i-m
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line length
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fix links
mark-i-m
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fix links
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Fix non-link

mark-i-m
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Joshua Nelson
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Escape other non-link

mark-i-m
and
Joshua Nelson
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fix links
mark-i-m
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