Document file-contents-sorter arguments

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 You must provide list of target files as input to it.
 Note that this hook WILL remove blank lines and does NOT respect any comments.
+The following arguments are available:
+- `--ignore-case` - fold lower case to upper case characters.
+- `--unique` - ensure each line is unique.
 #### `forbid-new-submodules`
 Prevent addition of new git submodules.