Ignore commonly generated Mac OS junk files

This is optional, but it would be a huge quality of life improvement for anyone developing on Mac OS, as the first thing I do when starting a new project is always to add this to the .gitignore. Example: Just from the small changes I've made to the build script alone, I've generated 4 .DS_Store files, which would clutter up this PR if I included them.

.gitignore

@@ -24,6 +24,10 @@ bin/
 .classpath
 .project
 
+# macos
+
+*.DS_Store
+
 # fabric
 
 run/