feat: Add Bootstrap 4 Theme support

packages/bootstrap-4/.gitignore

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+.rts2_**

packages/bootstrap-4/README.md

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+<!-- PROJECT LOGO -->
+<br />
+<p align="center">
+  <a href="https://github.com/cybertec-postgresql/rjsf-material-ui">
+    <img src="https://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/Bootstrap_logo.svg/800px-Bootstrap_logo.svg.png" alt="Logo" width="140" height="120">
+  </a>
+
+  <h3 align="center">@rjsf/bootstrap-4</h3>
+
+  <p align="center">
+  Bootstrap-4 theme, fields and widgets for <a href="https://github.com/mozilla-services/react-jsonschema-form/"><code>react-jsonschema-form</code></a>.
+    <br />
+    <a href="https://react-jsonschema-form.readthedocs.io/en/latest/"><strong>Explore the docs »</strong></a>
+    <br />
+    <br />
+    <a href="#">View Playground</a>
+    ·
+    <a href="#">Report Bug</a>
+    ·
+    <a href="#">Request Feature</a>
+  </p>
+</p>
+
+<!-- ABOUT THE PROJECT -->
+
+## About The Project
+
+
+Exports `bootstrap-4` theme, fields and widgets for `react-jsonschema-form`.
+
+### Built With
+
+- [react-jsonschema-form](https://github.com/mozilla-services/react-jsonschema-form/)
+- [Bootstrap-4](https://getbootstrap.com/docs/4.0/)
+- [Typescript](https://www.typescriptlang.org/)
+
+<!-- GETTING STARTED -->
+
+## Getting Started
+
+### Prerequisites
+
+- `react-bootstrap >= 1.0.1`
+- `@rjsf/core >= 2.2.0`
+
+```bash
+yarn add react-bootstrap @rjsf/core
+```
+
+### Installation
+
+```bash
+yarn add @rjsf/bootstrap-4
+```
+
+## Usage
+
+```js
+import Form from '@rjsf/bootstrap-4';
+```
+
+or
+
+```js
+import { withTheme } from '@rjsf/core';
+import { Theme as Bootstrap4Theme } from '@rjsf/bootstrap-4';
+
+const Form = withTheme(Bootstrap4Theme);
+```
+
+<!-- MARKDOWN LINKS & IMAGES -->
+<!-- https://www.markdownguide.org/basic-syntax/#reference-style-links -->
+
+[build-shield]: https://img.shields.io/circleci/build/github/cybertec-postgresql/rjsf-material-ui.svg?style=flat-square&token=a58b0890f96bff2b53eef0f4d9c9e5d16eec2200
+[build-url]: https://circleci.com/gh/cybertec-postgresql/rjsf-material-ui
+[contributors-shield]: https://img.shields.io/badge/contributors-1-orange.svg?style=flat-square
+[contributors-url]: https://github.com/cybertec-postgresql/rjsf-material-ui/graphs/contributors
+[license-shield]: https://img.shields.io/badge/license-MIT-blue.svg?style=flat-square
+[license-url]: https://choosealicense.com/licenses/mit
+[npm-shield]: https://img.shields.io/npm/v/rjsf-material-ui/latest.svg?style=flat-square
+[npm-url]: https://www.npmjs.com/package/rjsf-material-ui
+[npm-dl-shield]: https://img.shields.io/npm/dm/rjsf-material-ui.svg?style=flat-square
+[npm-dl-url]: https://www.npmjs.com/package/rjsf-material-ui
+[product-screenshot]: https://raw.githubusercontent.com/cybertec-postgresql/rjsf-material-ui/master/screenshot.png