Added readme

README.md

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+# arg - A simple command line argument parser
+This is a simple tool with a simple job: figure out the value of a single
+command line argument. Tools like `getopt` and `getopts` are much too
+complicated. It's much nicer to be able to write shell scripts like this:
+
+```
+#!/bin/sh
+dir=`arg --dir "$@" || arg -d "$@" || echo "$HOME/Downloads"`
+if arg -v "$@" || arg --verbose "$@"; then
+    echo "Downloading to $dir"
+fi
+cd $path && curl -O example.com/file.zip
+```
+
+## Usage
+`arg -f ...`, `arg -flag ...`, or `arg --flag ...`. If `arg` finds the
+given flag among the rest of the command line arguments, it will print
+the flag's value (if any) and exit successfully, otherwise, it will
+fail (exit status 1). The value may be of the form `--flag=value`,
+`--flag value`, `-f value`. Single-letter flags will match when grouped
+with other single letter flags, but will not have a value, i.e.
+`arg -b -abc foo` will exit successfully without printing anything.
+When using `arg` in a shell script, it is best to use quotes around `$@`,
+as in `arg --foo "$@"`, so arguments with spaces will parse properly,
+like `my_script.sh --flag "one two"`.