message: Clarify warning
  I found this confusing so I took the wording from customized it for context and added it to the `README` to avoid future confusion.
diff: @@ -137,7 +137,7 @@ custom configuration file (relative to the project's root directory)
  }
  }
  ```
  -**Warning: Using \_\_TS_CONFIG__ option in globals is deprecated and soon will be removed.**
  +**Warning: In the past, a `__TS_CONFIG__` section could be used to provide an inline TypeScript configuration. Going forward, all custom configurations should be in a different file and the `tsConfigFile` key in the `ts-jest` section (under `globals`) should point to this file (as shown above). You can ignore this if the configuration you want is identical to the main `tsconfig.json`.**
  For all available `tsc` options see [TypeScript docs](https://www.typescriptlang.org/docs/handbook/compiler-options.html).
  