Enhance README and update commit message hook logic (#19)

* Please set the OPENAI_API_KEY environment variable.
Update readme

* Fix check

.githooks/prepare-commit-msg

@@ -3,7 +3,7 @@ LC_ALL=C
 
 COMMIT_MSG_FILE=$1
 
-if [[ -z "$COMMIT_MSG_FILE" ]]; then
+if [[ -n "$COMMIT_MSG_FILE" ]]; then
     exit 0
 fi
 

README.md

@@ -1,3 +1,25 @@
 # ![GIT Hooks + OpenAI - Generate GIT commit messages from Open AI](docs/images/splash.png)
 
 🧠 🧰 This tool generates AI-powered commit messages via Git hooks, automating meaningful message suggestions from OpenAI and others to improve commit quality and efficiency.
+
+---
+
+## What does I do
+
+Generates a commit message based on `git diff` result using the [OpenAI](https://platform.openai.com/docs/overview) API.
+
+---
+
+## Getting started
+
+```ps
+dotnet tool install -g AiCommitMessage
+cd my-project/
+dotnet-aicommitmessage install-hook
+```
+
+1. Install the tool globally.
+2. Move to the project folder.
+3. Install the Git hook on the default `hooks` directory.
+
+---