Rename 'master' branch to 'main'

[mattt]

As discussed in #184 (comment).

[compnerd]

Just a note that this change will require a code change as well here:

swift-doc/Sources/swift-doc/Subcommands/Generate.swift

Line 163 in 307e486

let url = URL(string: "https://raw.githubusercontent.com/SwiftDocOrg/swift-doc/master/Resources/all.min.css")!

[mattt]

@compnerd Yep. Thanks for the reminder. We'll keep a master branch around for a while to make sure we don't break existing clients.

Prospectively, that code is a holdover from a migration to package resources that we had to back out of. Obviously it'd be better if the CSS was bundled with the tool; I'll give that another try, maybe for 1.0.0.

[Lukas-Stuehrk]

Also related to #231. Bundling the CSS with the tool will also fix this problem.