Add Swift Package Manager to Swift.gitignore

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

Swift.gitignore

@@ -27,6 +27,12 @@ xcuserdata
 *.hmap
 *.ipa
 
+# Swift Package Manager
+#
+# Add this line if you want to avoid checking in source code from Swift Package Manager dependencies.
+# Packages/
+.build/
+
 # CocoaPods
 #
 # We recommend against adding the Pods directory to your .gitignore. However