feat: add LICENSE & README

LICENSE

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+MIT License
+
+Copyright (c) 2023 Marcello Lamonaca
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# json2env
+
+Convert valid JSON to environment variables or an `.env`-line file.
+
+## Usage
+
+```sh
+JSON to Env Var converter
+
+Usage: json2env.exe [OPTIONS]
+
+Options:
+  -i, --input <FILE>        Input file, defaults to STDIN if not specified
+  -o, --output <FILE>       Output file, defaults to STDOUT if not specified
+  -s, --separator <STRING>  Separator for nested keys
+  -h, --help                Print help
+  -V, --version             Print version
+```
+
+## Installation
+
+You can either install the tool with `cargo`:
+
+```sh
+cargo install --path <path/to/repo>
+```
+
+or build the executable with (output in `target/release`):
+
+```sh
+cargo build --release
+```