doc: add note to contributor docs about warranted PR's

CONTRIBUTING.md

@@ -119,6 +119,8 @@ At this stage one should expect comments and review from other contributors. You
 can add more commits to your pull request by committing them locally and pushing
 to your fork until you have satisfied all feedback.
 
+Note: Code review is a burdensome but important part of the development process, and as such, certain types of pull requests are rejected. In general, if the **improvements** do not warrant the **review effort** required, the PR has a high chance of being rejected. It is up to the PR author to convince the reviewers that the changes warrant the review effort, and if reviewers are "Concept NAK'ing" the PR, the author may need to present arguments and/or do research backing their suggested changes.
+
 Squashing Commits
 ---------------------------
 If your pull request is accepted for merging, you may be asked by a maintainer