Contributing: Update contributing guide and use Markdown.

CONTRIBUTING.md

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+# Contributing code to Matrix
+
+Please read Synapse [contributing guide](https://github.com/matrix-org/synapse/blob/master/CONTRIBUTING.md).
+
+# Contributing code to Element iOS
+
+## I want to help translating
+
+If you want to fix an issue for an English string, please submit a pull request to the Element iOS GitHub repository.
+If you want to fix an issue for another language, add a missing translation, or  add a new language, please read [Element Web translating guide](https://github.com/vector-im/element-web/blob/develop/docs/translating.md) first and then use the Element iOS [Weblate](https://translate.riot.im/projects/riot-ios/).
+
+If you have any question regarding translations please ask in [Element Translation room](#element-translations:matrix.org).
+
+## Setting up a development environment
+
+Please refer to the [installation guide](INSTALL.md) to setup the project.
+
+## Implement a new screen or new screen flow
+
+New screen flows are currently using MVVM-Coordinator pattern. Please refer to the screen template [Readme](Tools/Templates/README.md) to create a new screen or a new screen flow.
+
+## Coding style
+
+For Swift coding style we use [SwiftLint](https://github.com/realm/SwiftLint) to check some conventions at compile time (rules are located in the `.swiftlint.yml` file). 
+Otherwise please have a look to [Apple Swift conventions](https://swift.org/documentation/api-design-guidelines.html#conventions). We are also using some of the conventions of [raywenderlich.com Swift style guide]([https://github.com/raywenderlich/swift-style-guide).
+
+## Pull request
+
+When you are making a pull request please read carefully the [Pull Request Checklist](https://github.com/vector-im/element-ios/blob/develop/.github/PULL_REQUEST_TEMPLATE.md).
+
+## Thanks
+
+Thanks for contributing to Matrix projects!