Add README.md

Author: Kissaki

custom-completions/md-to-clip/README.md

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+# `md-to-clip` completions
+
+Custom completions for the [`md-to-clip` command line tool](https://github.com/command-line-interface-pages/v2-tooling/tree/main/md-to-clip).
+
+## Usage
+
+Simply import the extern definitions with
+
+```nushell
+source path/to/md-to-clip-completions.nu
+```
+
+Afterwards, when you enter `md-to-clip -` and then press tab, the available flags will be displayed:
+
+```nushell
+nu ❯ | md-to-clip -
+--author                      Display author
+--email                       Display author email
+--help                        Display help
+--no-file-save                Whether to display conversion result in stdout instead of writing it to a file
+--output-directory            Directory where conversion result will be written
+--special-placeholder-config  Config with special placeholders
+--version                     Display version
+-a                            Display author
+-e                            Display author email
+-h                            Display help
+-v                            Display version
+```