docs: add new PDF Export optional package

Author: ghiscoding

README.md

@@ -25,6 +25,7 @@ Again the following dependencies are totally **OPTIONAL**
 | [@slickgrid-universal/composite-editor-component](https://github.com/ghiscoding/slickgrid-universal/tree/master/packages/composite-editor-component) | [![npm](https://img.shields.io/npm/v/@slickgrid-universal/composite-editor-component.svg?color=forest)](https://www.npmjs.com/package/@slickgrid-universal/composite-editor-component) | Composite Editor Modal Component |
 | [@slickgrid-universal/custom-tooltip-plugin](https://github.com/ghiscoding/slickgrid-universal/tree/master/packages/custom-tooltip-plugin) | [![npm](https://img.shields.io/npm/v/@slickgrid-universal/custom-tooltip-plugin.svg?color=forest)](https://www.npmjs.com/package/@slickgrid-universal/custom-tooltip-plugin) | Custom Tooltip Plugin |
 | [@slickgrid-universal/excel-export](https://github.com/ghiscoding/slickgrid-universal/tree/master/packages/excel-export) | [![npm](https://img.shields.io/npm/v/@slickgrid-universal/excel-export.svg?color=forest)](https://www.npmjs.com/package/@slickgrid-universal/excel-export) | Export to Excel Service (xls/xlsx) |
+| [@slickgrid-universal/pdf-export](https://github.com/ghiscoding/slickgrid-universal/tree/master/packages/pdf-export) | [![npm](https://img.shields.io/npm/v/@slickgrid-universal/pdf-export.svg?color=forest)](https://www.npmjs.com/package/@slickgrid-universal/pdf-export) | Export to PDF Service |
 | [@slickgrid-universal/text-export](https://github.com/ghiscoding/slickgrid-universal/tree/master/packages/text-export) | [![npm](https://img.shields.io/npm/v/@slickgrid-universal/text-export.svg?color=forest)](https://www.npmjs.com/package/@slickgrid-universal/text-export) | Export to Text File Service (csv/txt) |
 | [@slickgrid-universal/graphql](https://github.com/ghiscoding/slickgrid-universal/tree/master/packages/graphql) | [![npm](https://img.shields.io/npm/v/@slickgrid-universal/graphql.svg?color=forest)](https://www.npmjs.com/package/@slickgrid-universal/graphql) | GraphQL Query Service (support Filter/Sort/Pagination) |
 | [@slickgrid-universal/odata](https://github.com/ghiscoding/slickgrid-universal/tree/master/packages/odata) | [![npm](https://img.shields.io/npm/v/@slickgrid-universal/odata.svg?color=forest)](https://www.npmjs.com/package/@slickgrid-universal/odata) | OData Query Service (support Filter/Sort/Pagination) |