docs: update readme w/ experimental badge

Author: benmonro

README.md

@@ -107,7 +107,7 @@ module.exports = {
 | [prefer-empty](https://github.com/testing-library/eslint-plugin-jest-dom/blob/master/docs/rules/prefer-empty.md)                               | 👍  | 🔧  | Prefer toBeEmpty over checking innerHTML                                                         |
 | [prefer-enabled-disabled](https://github.com/testing-library/eslint-plugin-jest-dom/blob/master/docs/rules/prefer-enabled-disabled.md)         | 👍  | 🔧  | prefer toBeDisabled or toBeEnabled over checking attributes                                      |
 | [prefer-focus](https://github.com/testing-library/eslint-plugin-jest-dom/blob/master/docs/rules/prefer-focus.md)                               | 👍  | 🔧  | prefer toHaveFocus over checking document.activeElement                                          |
-| [prefer-in-document](https://github.com/testing-library/eslint-plugin-jest-dom/blob/master/docs/rules/prefer-in-document.md)                   |     | 🔧  | Prefer .toBeInTheDocument() in favor of checking the length of the result using .toHaveLength(1) |
+| [prefer-in-document](https://github.com/testing-library/eslint-plugin-jest-dom/blob/master/docs/rules/prefer-in-document.md)[![experimental](http://badges.github.io/stability-badges/dist/experimental.svg)](http://github.com/badges/stability-badges)                   |     | 🔧  | Prefer .toBeInTheDocument() in favor of checking the length of the result using .toHaveLength(1) |
 | [prefer-required](https://github.com/testing-library/eslint-plugin-jest-dom/blob/master/docs/rules/prefer-required.md)                         | 👍  | 🔧  | prefer toBeRequired over checking properties                                                     |
 | [prefer-to-have-attribute](https://github.com/testing-library/eslint-plugin-jest-dom/blob/master/docs/rules/prefer-to-have-attribute.md)       | 👍  | 🔧  | prefer toHaveAttribute over checking getAttribute/hasAttribute                                   |
 | [prefer-to-have-style](https://github.com/testing-library/eslint-plugin-jest-dom/blob/master/docs/rules/prefer-to-have-style.md)               | 👍  | 🔧  | prefer toHaveStyle over checking element style                                                   |