add link to test-custom-error-boundary

Author: bahmutov

README.md

@@ -115,6 +115,7 @@ Spec | Description
 [use-lodash-fp](cypress/component/basic/use-lodash-fp) | Imports and tests methods from `lodash/fp` dependency
 [document-spec](cypress/component/basic/document) | Checks `document` dimensions from the component
 [styled-components](cypress/component/basic/styled-components) | Test components that use [styled-components](https://www.styled-components.com/)
+[test-custom-error-boundary](https://github.com/bahmutov/test-custom-error-boundary) | Play with a component that implements error boundary
 <!-- prettier-ignore-end -->
 
 plus a few smaller sanity specs in [cypress/component/basic](cypress/component/basic) folder.