minor #4949 Fixed build readme on Windows fails if using Git Bash (Mintty) (8ctopus)

This PR was merged into the 2.15 branch.

Discussion
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Fixed build readme on Windows fails if using Git Bash (Mintty)

Git on Windows comes with Git Bash which uses Mintty.

In Git Bash the command to update the README.rst `php php-cs-fixer readme > README.rst` fails with error:

```
$ php php-cs-fixer readme > README.rst
stdout is not a tty
```

This modest contribution fixes it in Git Bash but also works in the regular command line.

Commits
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8e39a0a Fixed build readme on Windows fails if using Git Bash (Mintty)

Author: SpacePossum

CONTRIBUTING.md

@@ -13,7 +13,7 @@ If you need any help, don't hesitate to ask the community on [Gitter](https://gi
 * Make changes.
 * If you are adding functionality or fixing a bug - add a test! Prefer adding new test cases over modifying existing ones.
 * Make sure there is no trailing spaces in code: `./check_trailing_spaces.sh`.
-* Regenerate README: `php php-cs-fixer readme > README.rst`. Do not modify `README.rst` manually!
+* Regenerate README: `php php-cs-fixer readme > README.rst` (Windows `php.exe php-cs-fixer readme > README.rst`). Do not modify `README.rst` manually!
 * Check if tests pass: `vendor/bin/phpunit`.
 * Fix project itself: `php php-cs-fixer fix`.