Adds missing CONTRIBUTING file

CONTRIBUTING.md

@@ -0,0 +1,30 @@
+# Contributing to Zipkin
+
+If you would like to contribute code you can do so through GitHub by forking the repository and sending a pull request (on a branch other than `master` or `gh-pages`).
+
+When submitting code, please apply [Square Code Style](https://github.com/square/java-code-styles).
+* If the settings import correctly, CodeStyle/Java will be named Square and use 2 space tab and indent, with 4 space continuation indent.
+
+## License
+
+By contributing your code, you agree to license your contribution under the terms of the APLv2: https://github.com/openzipkin/zipkin/blob/master/LICENSE
+
+All files are released with the Apache 2.0 license.
+
+If you are adding a new file it should have a header like below. This can be automatically added by running `./mvnw com.mycila:license-maven-plugin:format`.
+
+```
+/**
+ * Copyright 2016 The OpenZipkin 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.
+ */
+ ```