doc: add policy for distribution

doc/contributing/distribution.md

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+# Node.js Distribution Policy
+
+This document describes some policies around what is and is not included in the
+Node.js distribution.
+
+## Inclusion
+
+Node.js includes many dependencies, such as V8, Undici and others; and comes
+bundled with standalone tools such as `npm`. It is not a goal of the Node.js
+project to provide a level playing field for all dependencies or bundled tools.
+With limited resources, the Node.js project does not intend to bundle or
+officially support multiple dependencies or tools that serve the same purpose.
+
+## Package managers
+
+The Node.js distribution will include `npm`, and will not include any other
+package managers.