docs: add explanation of why tabWidth defaults to 4 (#2002)

Author: oekazuma

README.md

@@ -169,7 +169,7 @@ Prettier for PHP supports the following options. We recommend that all users set
 | ------------------ | --------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `phpVersion`       | `"5.4"`   | Allows specifying the PHP version you're using. If you're using PHP 7.1 or later, setting this option will make use of modern language features in the printed output. If you're using PHP 5.3 or lower, you'll have to set this option or Prettier will generate incompatible code.                                                                                                  |
 | `printWidth`       | `80`      | Same as in Prettier ([see prettier docs](https://prettier.io/docs/en/options.html#print-width))                                                                                                                                                                                                                                                                                       |
-| `tabWidth`         | `4`       | Same as in Prettier ([see prettier docs](https://prettier.io/docs/en/options.html#tab-width))                                                                                                                                                                                                                                                                                         |
+| `tabWidth`         | `4`       | Same as in Prettier ([see prettier docs](https://prettier.io/docs/en/options.html#tab-width)), The default is `4` based on the `PSR-2` coding standard.                                                                                                                                                                                                                                                                                        |
 | `useTabs`          | `false`   | Same as in Prettier ([see prettier docs](https://prettier.io/docs/en/options.html#tabs))                                                                                                                                                                                                                                                                                              |
 | `singleQuote`      | `false`   | If set to `"true"`, strings that use double quotes but do not rely on the features they add, will be reformatted. Example: `"foo" -> 'foo'`, `"foo $bar" -> "foo $bar"`.                                                                                                                                                                                                              |
 | `trailingCommaPHP` | `true`   | If set to `true`, trailing commas will be added wherever possible. <br> If set to `false`, no trailing commas are printed.                                                                                                                                                                                                                                                          |