docs(contributing): signal intent behind authorship invitations

Keep maintainer sole discretion unchanged; add one clause naming the
spirit of the bar ("substantial intellectual contribution to the
library or to the paper itself") so potential collaborators know what
direction to aim without turning it into a rubric.

Author: brycewang-stanford

CONTRIBUTING.md

@@ -219,9 +219,10 @@ contributions — bug fixes, docs, tests, refactors, new estimators.
 ### Paper Authorship (by invitation)
 
 **By default, code contributors are not co-authors of StatsPAI
-papers.** The core maintainers reserve the sole right to extend
-authorship invitations to contributors whose work they judge to be
-exceptional. There is no fixed rubric — the decision rests with the
+papers.** Authorship is extended, at the maintainers' sole
+discretion, to contributors whose work rises to the level of
+substantial intellectual contribution to the library or to the paper
+itself. There is no fixed rubric; the decision rests with the
 maintainers and is made case by case.
 
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