Add contributing

CONTRIBUTING.md

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+# 💻 Contributing to DifferenceKit
+
+First of all, thanks for your interest in DifferenceKit. 
+
+There are several ways to contribute to this project. We welcome contributions in all ways. 
+We have made some contribution guidelines to smoothly incorporate your opinions and code into this project. 
+
+## 📝 Open Issue
+
+When you found a bug or having a feature request, search for the issue from the [existing](https://github.com/ra1028/DifferenceKit/issues) and feel free to open the issue after making sure it isn't already reported. 
+
+In order to we understand your issue accurately, please include as much information as possible in the issue template. 
+The screenshot are also big clue to understand the issue. 
+
+If you know exactly how to fix the bug you report or implement the feature you propose, please pull request instead of an issue. 
+
+## 🚀 Pull Request
+
+We are waiting for a pull request to make this project more better with us. 
+If you want to add a new feature, let's discuss about it first on issue. 
+
+```bash
+$ git clone https://github.com/ra1028/DifferenceKit.git
+$ cd DifferenceKit/
+$ open DifferenceKit.xcworkspace
+```
+
+### Lint
+
+Please introduce [SwiftLint](https://github.com/realm/SwiftLint) into your environment before start writing the code. 
+Xcode automatically runs lint in the build phase. 
+
+The code written according to lint should match our coding style, but for particular cases where style is unknown, refer to the existing code base. 
+
+### Test
+
+The test will tells us the validity of your code. 
+All codes entering the master must pass the all tests. 
+If you change the code or add new features, you should add tests. 
+
+### Documentation
+
+Please write the document using [Xcode markup](https://developer.apple.com/library/archive/documentation/Xcode/Reference/xcode_markup_formatting_ref/) to the code you added. 
+Documentation template is inserted automatically by using Xcode shortcut **⌥⌘/**. 
+Our document style is slightly different from the template. The example is below. 
+```swift
+/// The example class for documentation.
+final class Foo {
+ /// A property value.
+ let prop: Int
+
+ /// Create a new foo with a param.
+ ///
+ /// - Parameters:
+ /// - param: An Int value for prop.
+ init(param: Int) {
+ prop = param
+ }
+
+ /// Returns a string value concatenating `param1` and `param2`.
+ ///
+ /// - Parameters:
+ /// - param1: An Int value for prefix.
+ /// - param2: A String value for suffix.
+ ///
+ /// - Returns: A string concatenating given params.
+ func bar(param1: Int, param2: String) -> String {
+ return "\(param1)" + param2
+ }
+}
+```
+
+## [Developer's Certificate of Origin 1.1](https://elinux.org/Developer_Certificate_Of_Origin)
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+ have the right to submit it under the open source license
+ indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+ of my knowledge, is covered under an appropriate open source
+ license and I have the right under that license to submit that
+ work with modifications, whether created in whole or in part
+ by me, under the same open source license (unless I am
+ permitted to submit under a different license), as indicated
+ in the file; or
+
+(c) The contribution was provided directly to me by some other
+ person who certified (a), (b) or (c) and I have not modified
+ it.
+
+(d) I understand and agree that this project and the contribution
+ are public and that a record of the contribution (including all
+ personal information I submit with it, including my sign-off) is
+ maintained indefinitely and may be redistributed consistent with
+ this project or the open source license(s) involved.