Registering by extension is wrongly documented

[luzat]

README.md is using the following syntax for registering backend layouts of an extension:

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['BackendLayoutDataProvider'][$_EXTKEY]
= 'GeorgRinger\\BelayoutFileprovider\\Provider\\FileProvider';

The code currently seems to require:

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['BackendLayoutFileProvider']['ext'][] = $_EXTKEY;

The latter works for me. Note File vs. Data. and the different array structure. It might make sense to support

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['BackendLayoutFileProvider']['ext'][$_EXTKEY] = 'Resources/Private/BackendLayouts';

instead, but I don't really care.