README: refine Contribute section.

This adds a bit more colour to prerequiste 3 to disambiguate the "subjective perferences" term.

README.md

@@ -37,7 +37,7 @@ Lotus is a universally open project and welcomes contributions of all kinds: cod
 
 1. If the proposal entails a protocol change, please first submit a [Filecoin Improvement Proposal](https://github.com/filecoin-project/FIPs).
 2. If the change is complex and requires prior discussion, [open an issue](github.com/filecoin-project/lotus/issues) or a [discussion](https://github.com/filecoin-project/lotus/discussions) to request feedback before you start working on a pull request. This is to avoid disappointment and sunk costs, in case the change is not actually needed or accepted.
-3. Please refrain from submitting PRs to adapt existing code to subjective preferences.
+3. Please refrain from submitting PRs to adapt existing code to subjective preferences. The changeset should contain functional or technical improvements/enhancements, bug fixes, new features, or some other clear material contribution. Simple stylistic changes are likely to be rejected in order to reduce code churn.
 
 When implementing a change: