improvement(Core): added CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Persian Tools
+
+🎉 First off, thanks for taking the time to contribute! 🎉
+
+The following is a set of steps for contributing to Dart version of Persian tools package.
+
+## Proposing a Change
+
+If you intend to change the public API, or make any non-trivial changes
+to the implementation, we recommend `filing an issue`.
+This lets us reach an agreement on your proposal before you put significant
+effort into it.
+
+if you're fixing a bug you need to mention that issue in your PR. This helps us to understand exactly what you are working on.
+
+## Creating a Pull Request
+
+Before creating a pull request please:
+
+1. First of all make sure you have installed dart SDK on your machine. 
+1. Fork the repository.
+1. use `git clone` to clone the project.
+1. create your branch from `dev`. we use `master` branch only for production. all development changes should submitted to `dev` branch ( For the Branch name, please [Follow this Convention](https://dev.to/couchcamote/git-branching-name-convention-cch "Follow this Convention") ).
+1. Install all dependencies (`flutter packages get` or `pub get`).
+1. Squash your commits and ensure you have a meaningful commit message ( for the commit messages, please [follow this convention](https://www.conventionalcommits.org/en/v1.0.0/ "follow this convention") ).
+1. If you’ve fixed a bug or added code that should be tested, add tests!
+Pull Requests without 100% test coverage will not be approved.
+1. Ensure the test suite passes ( `dart test .` ).
+1. If you've changed the public API, make sure to update/add documentation.
+1. Format your code (`dart format .`).
+1. Analyze your code (`dartanalyzer --fatal-infos --fatal-warnings .`).
+1. Create the Pull Request.
+
+## Adding Documentation
+
+If you are creating/changing the public API please make sure that the public methods, classes and variables has documentation.
+You should also change the `README.md` file if the changes related to it.
+
+## Adding an example
+
+If you are creating/changing the public API, you should create/update example for it. example should placed in `/example` directory and the name of  the example file must be similar to the following name.
+`example/example_feature_name.dart`.
+
+## License
+
+By contributing to Bloc, you agree that your contributions will be licensed
+under its [MIT license](LICENSE).