Add changelog and contributing files

CHANGELOG.md

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+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [1.0.0] - 2018-07-19
+### Added
+- Initial commit

CONTRIBUTING.md

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+Contributing to the Elasticsearch Component
+====================================
+
+Thanks for getting interested in contributing to one of our projects. To help us the right way, here are some rules to follow.
+
+Report bug
+----------
+
+To report a bug, just open a new issue. Try to:
+
+- Be the more explicit as you can
+- Be careful on naming your issue: try to be clear
+- Use example, another project hosted on github may be an acceptable example
+- Provide a unit test that exposes the bug
+
+### Security issues
+
+Don't open an issue but send an email to **[email protected]**.
+
+Contribute
+----------
+
+### Open new discussion
+
+You may have a great idea. Do not hesitate to send it to us using the following rules:
+
+1. Prefix your issue title by `[RFC]`
+2. Describe your idea
+3. Illustrate with fake code example what it could be
+
+### Code Contribution
+
+1. Start from the `master` branch
+2. Verify that tests are green
+3. Comment the code
+5. Fix the codestyle of your code with the following command
+
+```
+php-cs-fixer.phar fix --config=.php_cs.dist
+```
+
+Release policy
+--------------
+
+The release policy of BiiG OSS was originally defined [here](https://github.com/biig-io/DictionaryBundle/issues/12).
+
+Here it is:
+
+1. With every release we should fill the [release page](https://github.com/biig-io/ElasticsearchComponent/releases)
+2. We should release maximum 1 week after some contribution is merged (except doc)
+3. We open an issue/milestone for the release so we have a track and date for it
+4. Every **contributor to the project & BiiGer** can make a release (others can ask for in a new issue)
+5. We should apply this to any projects of BiiG
+6. We must respect [semver](https://semver.org/)
+
+It may not be apply correctly so you can open an issue to notice it.
+
+License
+-------
+
+This library is under an MIT license. Any contribution you make on it is under the same MIT license.