chore(docs): add a contributing guide (#1815)

CONTRIBUTING.md

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+# Contribution Guidelines
+
+## Issues and Feature Requests
+
+If you got a question, bug report, or feature request, submit an issue to [our repository](https://github.com/DevExpress/devextreme-reactive). You should check the [list of active issues](https://github.com/DevExpress/devextreme-reactive/issues) first to avoid duplicates. 
+
+You can use the [Support Center](https://www.devexpress.com/ask) to contact us if you have an active DevExtreme license.
+
+## Contribute Code and Content
+
+Important: **Read Section 5 of the [LICENSE AGREEMENT](LICENSE.md#5-submission-of-contributions) before you submit any contribution.**
+
+Note that DevExtreme Reactive has a development schedule and we cannot accept every pull request from non-DevExpress contributors.
+
+However, you can fork the repository, modify the source code in your fork, and make your builds.
+
+To submit a pull request, follow these guidelines:
+
+1. Describe all applied changes in detail
+2. Make sure that all [automatic checks](README_DEVELOPERS.md#tests-and-ci) pass

README_DEVELOPERS.md

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+# README for Developers
+
+We use the [Fork-and-Branch workflow](http://blog.scottlowe.org/2015/01/27/using-fork-branch-git-workflow/). Start with [forking the repository](https://help.github.com/articles/fork-a-repo/), then create a branch to which you commit modifications.
+
+**Do not clone the main repository**
+
+[Create a pull request](https://help.github.com/articles/creating-a-pull-request-from-a-fork/) to the `master` branch when you are ready to submit your changes. Each pull request should pass all automatic checks and at least one member of the DevExtreme team should [review](https://help.github.com/articles/about-pull-request-reviews/) it.
+
+Make sure that you properly configure your Git [username](https://help.github.com/articles/setting-your-username-in-git) and [email](https://help.github.com/articles/setting-your-email-in-git).
+
+## Install Required Software
+
+To set up a build environment, you need the following software installed:
+
+- [Node.js](https://nodejs.org/en/download/) and [yarn](https://yarnpkg.com/en/) (or [npm](https://www.npmjs.com/get-npm)). We recommend the LTS versions.
+- A web browser for development. We recommend Google Chrome.
+
+## Building
+
+Run the following command to install packages:
+
+    yarn
+
+or
+
+    npm install
+
+After installation, the following commands are available:
+
+- `yarn lint` or `npm run lint` - Executes linters
+- `yarn test` or `npm run test` - Runs tests
+- `yarn test:watch` or `npm run test:watch` - Runs an interactive test watcher
+- `yarn build`  or `npm run build` - Builds DevExtreme Reactive
+- `yarn build:watch`  or `npm run build:watch` - Builds DevExtreme Reactive in watch mode and runs demos. The following demo URLs are accessible:
+
+  - `http://localhost:3002/` - Grid demos
+  - `http://localhost:3004/` - Chart demos
+  - `http://localhost:3005/` - Scheduler demos
+
+We use a [monorepo](https://en.wikipedia.org/wiki/Monorepo), and you can execute the described commands from individual packages (`./packages/dx-react-core/`, `./packages/dx-react-grid/`, and so on).
+
+## Tests and CI
+
+We adhere to the [test-driven development](https://en.wikipedia.org/wiki/Test-driven_development) paradigm. It follows a simple rule: each PR should contain a test if possible. It means that if a PR fixes a bug, it should contain a [regression test](https://en.wikipedia.org/wiki/Regression_testing); if a PR implements a new functionality, it should contain tests for it, and so on.
+
+We recommend running tests and executing linters locally before you submit a PR.
+
+## Commit Message & Pull Request Title Guidelines
+
+We use the [Conventional Commits](https://www.conventionalcommits.org) specification for commit messages and PR titles. You should make sure that your PR also follows it.