message: Improve coverage section in README
diff: @@ -316,7 +316,7 @@ Runs on all [PHP versions supported](#tada-php-support "List of PHP versions sup
  coverage: xdebug
  ```
  -- The latest version of Xdebug compatible with the PHP version is set up by default.
  +- When you specify `coverage: xdebug`, the latest version of Xdebug compatible with the PHP version is set up by default.
  - If you need Xdebug 2.x on PHP 7.2, 7.3 or 7.4, you can specify `coverage: xdebug2`.
  ```yaml
  @@ -327,6 +327,8 @@ Runs on all [PHP versions supported](#tada-php-support "List of PHP versions sup
  coverage: xdebug2
  ```
  +**Note**: Xdebug is enabled by default on Ubuntu GitHub Actions images, so if you are not using it in your workflow it is recommended to disable it as that will have a positive impact on your PHP performance. Please refer to the [disable coverage](#disable-coverage) section for details.
  +
  ### PCOV
  Specify `coverage: pcov` to use `PCOV` and disable `Xdebug`.
  @@ -360,7 +362,6 @@ Specify `coverage: none` to disable both `Xdebug` and `PCOV`.
  Disable coverage for these reasons:
  - You are not generating coverage reports while testing.
  -- It will disable `Xdebug`, which will have a positive impact on PHP performance.
  - You are using `phpdbg` for running your tests.
  - You are profiling your code using `blackfire`.
  - You are using PHP in JIT mode. Please refer to [JIT configuration](#jit-configuration) section for more details.
  