docs: Add 'What this library can't test' section (#575)

Co-authored-by: Kent C. Dodds <me+github@kentcdodds.com>

Author: kalpeshsingh

docs/intro.md

@@ -39,6 +39,21 @@ is also a plugin to use testing-library queries for end-to-end tests in
 `DOM Testing Library` works with any environment that provides DOM APIs, such as
 Jest, Mocha + JSDOM, or a real browser
 
+### What you should avoid with Testing Library
+
+Testing Library encourages you to avoid testing
+[implementation details](https://kentcdodds.com/blog/testing-implementation-details)
+like the internals of a component you're testing (though it's still possible).
+[The Guiding Principles](/docs/guiding-principles) of this library emphasize a
+focus on tests that closely resemble how your web pages are interacted by the users.
+
+You may want to avoid following implementation details:
+
+1. Internal state of a component
+1. Internal methods of a component
+1. Lifecycle methods of a component
+1. Child components
+
 [jest]: https://jestjs.io
 [jsdom]: https://github.com/jsdom/jsdom
 [dom]: dom-testing-library/intro.md