tranlate CONTRIBUTING.md in English and rename the original contrib…

…ution guide `CONTRIBUTING.ja.md`

CONTRIBUTING.md

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+Contribution Guide
+======================
+
+PacketProxy is an open-source project and welcomes contributors to take part in the development, hopefully with comfort. To achieve this, contributing to this project currently requires no strict and uncomfortable rules.
+
+Any question or suggestion will, even if you feel it trivial, be appreciated and feel free to submit it as an `issue`.
+
+Currently no mailing list, IRC, slack, nor Gitter channel for this project exists, and we don't have a plan to have one. Sorry for the inconvenience.
+
+Fixing bugs or improving PacketProxy
+--------------------------------------
+
+If you'd like to merge changes you made on your fork to fix bugs or improve PacketProxy in any form, feel free to send pull requests.
+
+Currently we don't have any coding style guidelines and don't have any intension to require contributors to comply conding conventions.
+
+License
+-----------
+
+PacketProxy is an open-source project and its source code is distributed under Apache License 2.0. Therefore sending pull requests to this repository means you agreed the code you wrote is also distributed under the license. To avoid license violations, if your pull request contains any part of other software, please inform us.
+
+Contributioin License Agreement (CLA)
+-----------------------------------------
+
+All contributors must consent to a contribution licennse agreement (CLA) before contributions is accepted. Before sending a pull requests, take a look at [the CLA](https://dena.github.io/cla/).