Adding support for '+' to append one AttributedString to another makes building strings easier

[dhoerl]

public func + (left: NSMutableAttributedString, right: NSMutableAttributedString) -> NSMutableAttributedString { left.appendAttributedString(right) return left }

You can see all in my fork: https://github.com/dhoerl/string-in-chain

I also edited your readme to make the text a bit clearer (I speak German but could never have done as good a job in that language as you did with the readme!)

[lsolniczek]

Great work! Can you make Pull request with your updates to library? (one pull request for every update)