The amount of people who read through a README is probably in the single digit percentage of users who post issues (judging by the fact that so many issues have their resolution in part of the README).
The number of people (especially those who post issues, not PRs necessarily) who read a contribution guide is probably less than 1%.
GitHub issue templates can help in that regard, but it's still pretty minimal the amount of work 80% of people who post issues will put in towards solving it themselves first.
Normally if the README contains the restrictions on contribution and PRs right at the top I think it should catch everyone’s attention (well, hopefully…).
IMHO, the ideal solution is for GitHub to allow restrictions on issues and PRs for public repositories. You could always make the repo private though. I don’t think there is a restriction on private repo’s right now in GitHub.
The number of people (especially those who post issues, not PRs necessarily) who read a contribution guide is probably less than 1%.
GitHub issue templates can help in that regard, but it's still pretty minimal the amount of work 80% of people who post issues will put in towards solving it themselves first.