* added files to be ignored with swift package manager

* added comment for swiftpm and why it might want to be included in .gitignore

Adding Code Injection

Adding a comment in case one wants to ignore .xcworkspace

The whole screenshots folder shouldn't be ignore since there could be frameit configuration in it (Framefile.json, fonts, background.jpg, *.strings, etc...). It'd be better to just ignore the generated PNG screenshots.

The Objective-C and Swift gitignore files are missing `*.xccheckout`
and `*.xcscmblueprint`, but these are still included in the Xcode
gitignore. This commit synchronises the three files.

The files themselves contain information about the user's local
source control status and shouldn't be included in most Xcode
repositories.

`*.xcuserstate` appears to always be contained in `xcuserdata/`
which is already ignored.

Fastlane now has a mono repo, the documentation moved into the folder fastlane/docs

Matches the style used for Carthage - build disabled by default, checkouts optionally disabled.

These files are automatically generated by Xcode and maintain
information regarding source control. Xcode is typically used
in Objective-C and Swift projects, so add these to the ignored files for
these platforms.

some details based on #1221

I just copied Objective-C.gitignore. It seemed to do the job with a small Swift
project.

This got changed recently.

xcuserstate files are generated from within Xcode and saved in your project bundle to remember your last opened file, the open state of any group folders, open tabs, and any other user setting your project might need to remember

We shouldn't be ignoring these, it's down to the user and we shouldn't enforce them to ignore the `Pods/` directory

http://guides.cocoapods.org/using/using-cocoapods.html#should-i-ignore-the-pods-directory-in-source-control?

The pattern `*.xccheckout` is already ignored on line 15.