If you want to contribute code to DNS66, you are more than welcome to do so. DNS66 uses GitHub pull requests for code contributions.
A pull request should consist of a series of logical commits leading from a point in the master branch to whatever you are implementing. The pull request should not contain any fixup commits -- rebase your branch instead, and squash the fixup commits into their respective commits.
Make sure you have good commit messages. In particular, the first line should be a very short description, followed by an empty line, followed by one or more paragraphs explaining the change. The last paragraph should mention any github issues being fixed in a syntax understood by github.
If you squash changes together during a rebase, edit the commit message to describe the squashed commit as a whole - do not just stick the commit messages together.
Code is usually automatically formatted with Android Studio using the default coding style. Sometimes it is forgotten, but please try to make sure to follow the same coding style for any of your contributions.
Any code contribution will be considered to grant an implicit license under the GPL, version 3 or later, as stated below, or (on special exceptions) a different license explicitly included in the contribution that is compatible to the GPL 3.
Icons are welcome too. Modifications of existing icons shall have the same license as the modified file. New icons may be contributed under the GPL, version 3 or later (as below), or the Apache license, version 2.