setup new feedstocks to only push binaries for changes in the master branch

I've created a [PR](https://github.com/conda-forge/scandir-feedstock/pull/1) today for an version update.

Since I'm the maintainer and I felt lazy I directly pushed my new branch with the version changes to the feedstock repository and created a PR. This triggered the CI builds twice , for the push and the PR. The push CI then already updated the packages published on the conda-forge channel. The change was small and everything worked but this could have easily broken the package.

I think it would be nice to configure the default CI settings that only the master branch get's pushed to conda-forge to avoid this type of accidents for new maintainers. Or at least write a explanation about this to all maintainers for new feedstocks with big warnings signs.