Add new option to readme and fix flake8 error

Line length to long for help and add command line help with
--modified-files to README

README.rst

@@ -377,6 +377,9 @@ The following `command line arguments`_ can also be used to modify the defaults:
        should be supported by Black's output. [default: per-file auto-detection]
 -W WORKERS, --workers WORKERS
        How many parallel workers to allow, or ``0`` for one per core [default: 1]
+-m, --modified-only
+       Only consider files with modified content for checking. Skip moved or renamed
+       files.
 
 To change default values for these options for a given project,
 add a ``[tool.darker]`` or ``[tool.black]`` section to ``pyproject.toml`` in the

src/darker/help.py

@@ -147,4 +147,7 @@ def get_extra_instruction(dependency: str) -> str:
 
 WORKERS = "How many parallel workers to allow, or `0` for one per core [default: 1]"
 
-MODIFIED_ONLY = "Only consider files with modified content for checking. Skip moved or renamed files."
+MODIFIED_ONLY = (
+    "Only consider files with modified content for checking."
+    " Skip moved or renamed files."
+)