Update Unity.gitignore

Added OSX only files to the .gitignore, which is helpful for those developing on a Mac/OSX machine.

Additionally, a nice warning that you should convert to metafiles (not many new developers will know this) and is a helpful reminder.

Unity.gitignore

@@ -1,6 +1,17 @@
+##################
+# Unity ignores:
+#
+# !!! WARNING !!!
+#
+# … you MUST convert Unity to using Metafiles *before* you start using this
+#  .gitignore file, or you WILL lose data!
+#
+
 [Ll]ibrary/
 [Tt]emp/
 [Oo]bj/
+[Bb]uilds/
+[Ss]creenshots/
 
 # Autogenerated VS/MD solution and project files
 /*.csproj
@@ -11,3 +22,9 @@
 /*.userprefs
 /*.pidb
 /*.booproj
+
+# OS X only:
+.DS_Store
+*.swp
+*.Trashes
+