The github readme (https://github.com/mongodb/mongoid/) currently points users to Jira for creating issues but does not say anything about how to contribute code to Mongoid. In https://jira.mongodb.org/browse/MONGOID-5081 contribution guidelines were requested.

Some things I can think of right now are:

Changes should be applied to master branch first
Changes must have adequate test coverage
Backports to stable branches (7.0-7.2) are generally done if the change is a bug fix, it wouldn't break applications and the diff applies to the stable branch
6.x is generally not changed any more with exception of security vulnerabilities
5.x is generally not supported