chore: update readme.

Author: ya7010

README.md

@@ -10,10 +10,26 @@
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 `cmdcomp` generate command shell completion file (`bash` or `zsh`) from config
-`json`/`yaml`/`toml` file.
+`JSON`/`YAML`/`TOML` file.
 
 ![image](./docs/images/image.png)
 
+## Why was `cmdcomp` created?
+
+Completion files for many cli tools (`aws`, `gcloud`, etc.) are provided by
+self. However, if you want to use these cli tools in multiple products and
+multiple environments (prd, dev, etc.), it would be useful to be able to switch
+contexts easily. In this situation, I have a simple Shell Script wrapper to
+switches settings for each environment easily. (like `mycli prd aws s3 ...`).
+
+`cmdcomp` can generate completion files for your shell script using
+configuration wittened by `YAML` or `TOML` or `JSON`.
+
+In today's development, more and more cli tools be used.
+
+If you want to generate a completion file for a simple wrapper script, `cmdcomp`
+will be of great help.
+
 ## Install
 
 ```shell
@@ -23,7 +39,7 @@ pip install cmdcomp
 ## Usage
 
 ```shell
-cmdcomp --file $YOUR_CONFIG_FILE --shell-type bash
+cmdcomp --config $YOUR_CONFIG_FILE --shell-type bash
 ```
 
 ## Documentation