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 some external projects that the Node.js team does not maintain.
+The fact of a project's inclusion should not imply anything about the project
+relative to its competitors; in some cases, a project was added when it had no
+competitors. While the Node.js team supports and encourages competition in the
+JavaScript ecosystem, as a policy, the Node.js project does not include multiple
+dependencies or tools that serve the same purpose.
+
+The following user-accessible external projects are the ones chosen for their
+particular purposes:
+
+* JavaScript engine: V8
+* Package manager: `npm`
+
+Being user-accessible, removal or replacement of these projects could happen
+only as a semver-major change. In addition, Node.js includes external projects
+as internal dependencies. These may be replaced or removed at any time, provided
+that doing so is not a breaking change.