docs: add installation section to README

Author: sup3x

README.md

@@ -11,12 +11,42 @@ A zero-config CLI that parses your Git log and renders a contribution heatmap po
 
 ---
 
-## Quick Start
+## Installation
+
+**Run directly (no install needed):**
+```bash
+npx git-poster
+```
 
+**Install globally:**
 ```bash
+npm install -g git-poster
+git-poster
+```
+
+**Install locally in a project:**
+```bash
+npm install --save-dev git-poster
 npx git-poster
 ```
 
+**PNG support** (optional):
+```bash
+npm install -g sharp
+git-poster --png
+```
+
+> Requires Node.js 18 or later.
+
+---
+
+## Quick Start
+
+```bash
+cd your-repo
+git-poster
+```
+
 That's it. Run from any Git repository and get `git-poster.svg` in the current directory.
 
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