[llvm] Document the requirement that commits have a public email address (PR #109318)

[Reid Kleckner via llvm-commits]

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@@ -86,6 +86,11 @@ that notices of confidentiality or non-disclosure cannot be respected.
 Making and Submitting a Patch
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+The LLVM community requires contributors to have a public email address
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rnk wrote:

Should this be the first paragraph of the section? How about putting it after the non-disclosure notice paragraph, or immediately before?

https://github.com/llvm/llvm-project/pull/109318