added ability to revert (#21)

CONTRIBUTING.md

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+# Contributing
+
+- Love your input! Contributing to this project should be as easy and transparent as possible, whether it's:
+  - Reporting a bug
+  - Discussing the current state of the code
+  - Submitting a fix
+  - Proposing new features
+  - Becoming a maintainer
+
+## Pull Request Process
+
+- [GitHub flow](https://guides.github.com/introduction/flow/index.html) is used
+- Your pull requests are actively welcomed.
+- The steps:
+  1. Fork the repo and create your branch from master.
+  2. If you've added code that should be tested, add tests.
+  3. If you've changed APIs, update the documentation.
+  4. Ensure the test suite passes.
+  5. Make sure your code lints.
+  6. Issue that pull request!
+- 🙏 DO
+  - Document your changes in the pull request
+  - 💡 Check [developer notes](./docs/developer-notes.md) if you need help
+- ❗ DON'T
+  - Do not update the versions, current version is only [set by the maintainer](./docs/gitops.png) and updated automatically by [bump-everywhere](https://github.com/undergroundwires/bump-everywhere)
+
+## Guidelines
+
+### Extend scripts
+
+- Create a [pull request](./../CONTRIBUTING.md#Pull+Request+Process) for [application.yaml](./../src/application/application.yaml)
+- 🙏 For any new script, try to add `revertCode` that'll revert the changes caused by the script.
+- See [typings](./../src/application/application.yaml.d.ts) for documentation as code.
+
+### Handle the state in presentation layer
+
+- There are two types of components:
+  - **Stateless**, extends `Vue`
+  - **Stateful**, extends [`StatefulVue`](./src/presentation/StatefulVue.ts)
+    - The source of truth for the state lies in [application layer](./src/application/state) and must be updated from the views if they're mutating the state
+    - They mutate or/and reacts to changes in [application state](./src/application/state).
+    - You can react by getting the state and listening to it and update the view accordingly in [`mounted()`](https://vuejs.org/v2/api/#mounted) method.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under its GNU General Public License v3.0.