added license information

CONTRIBUTING.md

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+# Contributing to Nebula
+
+If you would like to contribute code you can do so through GitHub by forking the repository and sending a pull request. When submitting code, please make every effort to follow existing conventions and style in order to keep the code as readable as possible.
+
+## License
+
+By contributing your code, you agree to license your contribution under the terms of the [Apache License v2.0](http://www.apache.org/licenses/LICENSE-2.0). Your contributions should also include the following header:
+
+```
+/**
+ * Copyright 2015 the original author or authors.
+ * 
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+ ```