Add tsconfig.json include/files property warning to README

[13rac1]

If the "files" or "include" properties are set in the tsconfig.json file, then the setupJest.ts file must be included. Otherwise, the TypeScript compiler will fail to find fetchMock.

Example in tsconfig.json:

  "include": [
    "src/**/*",
    "test/**/*"
  ],

For fun: It'll work sometimes. I have a 100% reliable example where the existence of a newline at the top of the test file causes it to pass. Removing the newline causes it to fail.

Example failure:

$ jest
 FAIL  test/hello.test.ts
  ● Test suite failed to run

    TypeScript diagnostics (customize using `[jest-config].globals.ts-jest.diagnostics` option):
    test/hello.test.ts:48:5 - error TS2304: Cannot find name 'fetchMock'.

    48     fetchMock.mockResponseOnce('Hello');
           ~~~~~~~~~
    test/hello.test.ts:67:12 - error TS2304: Cannot find name 'fetchMock'.

    67     expect(fetchMock).toBeCalledTimes(1);
                  ~~~~~~~~~

Test Suites: 1 failed, 1 total
Tests:       0 total
Snapshots:   0 total
Time:        1.451s
Ran all test suites.

[13rac1]

No response in almost four months. Closing.