Add .swp files to .gitignore (swift-server#16)

Motivation:

This PR closes issue swift-server#1.
We do not want Vim's temporary .swp files in our repository.

Modifications:

* added .swp files to .gitignore

Result:

Git will ignore .swp files

Author: felixschlegel

.gitignore

@@ -8,3 +8,4 @@ DerivedData/
 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
 .netrc
 Package.resolved
+.*.sw?