swift-package-manager

Let’s name our files consistently

1. It is not necessary to repeat Package Manager in the URL as it is already part of the url, compare:
    * https://github.com/apple/swift-package-manager/blob/master/Documentation/Package-Manager-Community-Proposal.md
    * https://github.com/apple/swift-package-manager/blob/master/Documentation/CommunityProposal.md

2. None of our other Documentation files used hyphens.