swift-format: add a .swift-format file

This adds a .swift-format to tweak some of the default behaviours of
swift-format.  Although it isn't exactly the style used by this project,
it brings it closer.

Author: compnerd

.swift-format

@@ -0,0 +1,9 @@
+{
+  "version": 1,
+  "rules": {
+    "AllPublicDeclarationsHaveDocumentation": true,
+    "AlwaysUseLowerCamelCase": false,
+    "UseShorthandTypeNames": false,
+    "ValidateDocumentationComments": true
+  }
+}