ignore '.directory' (#2158)

.gitignore

@@ -5,6 +5,7 @@ node_modules
 .project
 .settings/
 npm-debug.log
+.directory
 
 # vim temporary files
 .*.sw[op]

CONTRIBUTING.md

@@ -14,6 +14,7 @@ There are a few preferences regarding **code contributions**:
 - vis.js follows the node.js code style as described
   [here](http://nodeguide.com/style.html).
 - When implementing new features, please update the documentation accordingly.
+- Make changes in the `develop` branch, not the `master` branch.
 - Send pull requests to the `develop` branch, not the `master` branch.
 - Only commit changes done in the source files under `lib`, not to the builds
   which are located in the folder `dist`.