Add _redirects to site include

Author: mattt

_config.yml

@@ -9,7 +9,7 @@ twitter_username:   SwiftDocOrg
 github_username:    SwiftDocOrg
 swift_version:      "Swift version 4.2 & previous"
 
-include:            [ "_[A-Z]*", "__[A-Z]*" ]
+include:            [ "_[A-Z]*", "__[A-Z]*", "_redirects" ]
 exclude:            [ "README.md", "LICENSE", "CC-BY-NC-LICENSE.md", "Gemfile", "Gemfile.lock", "Procfile" ]
 keep_files:         [ ".nojekyll", ".git", "CNAME", "README.md" ]