add README file

README.md

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+# Usage
+
+```
+./nft-edit-ruleset [--yes] [--fail] [--timeout NUM]
+```
+
+# Description
+
+Interactively edit the current nftables ruleset using the editor specified
+by environment variable EDITOR (defaulting to vim). Optionally, revert
+changes after a timeout given in seconds.
+
+# Options
+
+* `-f`, `--fail`: Exit unsuccessfully if changes fail to apply.
+* `-h`, `--help`: Display this message and exit.
+* `-t NUM`, `--timeout NUM`: Revert changes after NUM seconds.
+* `-y`, `--yes`: No confirmation before applying changes.
+
+# Exit Codes
+
+* 0: Success: No changes to apply or changes applied successfully.
+* 1: Error: Usage error or failed or aborted changes.
+
+# Author and License
+
+* Author: Tilman Kranz <tilt@linuxfoo.de>
+* License: MIT License [https://opensource.org/licenses/MIT](https://opensource.org/licenses/MIT)
+