Add README

Author: CeleritasCelery

README.md

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+# Blacken
+
+Use the python `black` package to reformat your python buffers.
+
+## usage
+
+The whole buffer can be reformatted with `blacken-buffer`. If you want
+to format every time you save, enable `blacken-mode` in relevant
+python buffers. Note that if `blacken-only-if-project-is-blackened` is
+non-nil, then blacken will only run if your `pyproject.toml` contains
+the `[tool.black]` setting. This setting is off by default.
+
+
+## customization
+
+The following options change the behavior of black when reformatting buffers.
+
+- `blacken-allow-py36` Allow using Python 3.6-only syntax on all input files.
+- `blacken-skip-string-normalization` Don't normalize string quotes or prefixes.
+- `blacken-fast-unsafe` Skips temporary sanity checks.
+- `blacken-line-length` Max line length enforced by blacken.