Add vfg-field-matrix into the readme

Author: shwld

README.md

@@ -245,6 +245,7 @@ This way, it will be easier for everyone to find it. Thank you !
 * [vfg-field-sourcecode](https://github.com/gwenaelp/vfg-field-sourcecode) - A source code field for vue-form-generator
 * [vfg-field-array](https://github.com/gwenaelp/vfg-field-array) - A vue-form-generator field to handle arrays of items of any type.
 * [vfg-field-object](https://github.com/gwenaelp/vfg-field-object) - A vue-form-generator field to handle objects, with or without schemas.
+* [vfg-field-matrix](https://github.com/shwld/vfg-field-matrix) - A matrix field for vue-form-generator.
 
 ## Contribution