Swift: ignore .swiftpm (#3083)

* added files to be ignored with swift package manager

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

Swift.gitignore

@@ -38,6 +38,12 @@ playground.xcworkspace
 # Packages/
 # Package.pins
 # Package.resolved
+# *.xcodeproj
+#
+# Xcode automatically generates this directory with a .xcworkspacedata file and xcuserdata
+# hence it is not needed unless you have added a package configuration file to your project
+# .swiftpm
+
 .build/
 
 # CocoaPods