docs: improve the usage instructions

README.md

@@ -1,18 +1,36 @@
-# gh-compare
+## gh-compare
 
-This is [`gh`](https://cli.github.com/) extension to replace the [`compare`](https://hub.github.com/hub-compare.1.html) command in the mostly deprecated tool, [`hub`](https://hub.github.com/).
+This is a [`gh`](https://cli.github.com/) extension to replace the [`compare`](https://hub.github.com/hub-compare.1.html) command in the mostly deprecated tool, [`hub`](https://hub.github.com/).
 
-## Installation
+### Installation
 
 ```shell
 gh extension install wassimk/gh-compare
 ```
 
-## Usage
+### Usage
+
+Compare the current branch against the `main` branch.
 
 ```shell
 gh compare
+```
+
+Compare the current branch against another branch.
+
+```shell
 gh compare other_branch
-gh compare main...my-branch
+```
+
+Compare any two branches.
+
+
+```shell
+gh compare their_branch...my_branch
+```
+
+Compare any two commits.
+
+```shell
 gh compare 7a67154..205b073
 ```