message: Fix links to type definition in Jest-Circus README.md
diff: +[type-definitions]: https://github.com/facebook/jest/blob/master/packages/jest-types/src/Circus.ts
  +
  <h1 align="center">
  <img src="https://jestjs.io/img/jest.png" height="150" width="150"/>
  <img src="https://jestjs.io/img/circus.png" height="150" width="150"/>
  Circus is a flux-based test runner for Jest that is fast, maintainable, and simple to extend.
  -Circus allows you to bind to events via an optional event handler on any [custom environment](https://jestjs.io/docs/configuration#testenvironment-string). See the [type definitions](https://github.com/facebook/jest/blob/master/packages/jest-circus/src/types.ts) for more information on the events and state data currently available.
  +Circus allows you to bind to events via an optional event handler on any [custom environment](https://jestjs.io/docs/configuration#testenvironment-string). See the [type definitions][type-definitions] for more information on the events and state data currently available.
  ```js
  import {Event, State} from 'jest-circus';
  @@ -28,7 +30,7 @@ class MyCustomEnvironment extends NodeEnvironment {
  Mutating event or state data is currently unsupported and may cause unexpected behavior or break in a future release without warning. New events, event data, and/or state data will not be considered a breaking change and may be added in any minor release.
  -Note, that `jest-circus` test runner would pause until a promise returned from `handleTestEvent` gets fulfilled. **However, there are a few events that do not conform to this rule, namely**: `start_describe_definition`, `finish_describe_definition`, `add_hook`, `add_test` or `error` (for the up-to-date list you can look at [SyncEvent type in the types definitions](https://github.com/facebook/jest/tree/master/packages/jest-types/src/Circus.ts)). That is caused by backward compatibility reasons and `process.on('unhandledRejection', callback)` signature, but that usually should not be a problem for most of the use cases.
  +Note, that `jest-circus` test runner would pause until a promise returned from `handleTestEvent` gets fulfilled. **However, there are a few events that do not conform to this rule, namely**: `start_describe_definition`, `finish_describe_definition`, `add_hook`, `add_test` or `error` (for the up-to-date list you can look at [SyncEvent type in the types definitions][type-definitions]). That is caused by backward compatibility reasons and `process.on('unhandledRejection', callback)` signature, but that usually should not be a problem for most of the use cases.
  ## Installation
  