Add README

Author: sankichi92

README.md

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+# search-once
+
+A command line application to search multiple websites at once.
+
+## Installation
+
+```console
+$ cargo install search-once
+```
+
+## Usage
+
+```console
+$ search-once --help
+A tool to search multiple websites at once
+
+Usage: search-once [OPTIONS] <QUERY>
+
+Arguments:
+  <QUERY>
+
+Options:
+  -c, --config <FILE>
+  -h, --help           Print help
+  -V, --version        Print version
+```
+
+## Configuration
+
+You can customize search sites via a YAML configuration file with the following format:
+
+```yaml
+sites:
+  - name: GitHub Rust repos
+    url: https://github.com/search?q=language%3ARust+%s&type=repositories
+  - name: Crates.io
+    url: https://crates.io/search?q=%s
+```
+
+The `%s` placeholder in the URL will be replaced with your query.
+
+The default configuration file will be automatically placed in the appropriate path based on your OS. You can find the path by running `search-once` without the `--config` option.