docs(readme): add note about fetch-depth (#25)

* Update README.md

Updating README.md to explicitly mention that fetch-depth is a key argument to checkout repo, otherwise this action is almost unusable.

* Update README.md

Usage of git admonition syntax and some edits on documentation.

Author: Gnyblast

README.md

@@ -19,6 +19,20 @@ This action generates a changelog based on your Git history using [git-cliff](ht
 
 - `OUTPUT`: Output file. (Default: `"git-cliff/CHANGELOG.md"`)
 
+> [!IMPORTANT]
+> It is important to checkout the repositories as:
+>  
+>  ```yaml
+>  - name: Checkout
+>    uses: actions/checkout@v4
+>    with:
+>      fetch-depth: 0
+>  ```
+>  
+>  `fetch-depth: 0` the is key input here to checkout action.
+>  
+>  Otherwise, you might end-up getting empty changelogs or `git ref` errors depending on arguments passed to `git-cliff`.
+
 ### Examples
 
 #### Simple