feat: add note about new branch_name token (#39)

Author: ndom91

content/docs/features/virtual-branches/commits.mdx

@@ -65,6 +65,7 @@ Custom prompts can contain three variables which we will replace with the approp
 - `%{emoji_style}` - Instructs the LLM whether or not to make use of [GitMoji](https://gitmoji.dev) in the title prefix, based on your settings.
 - `%{brief_style}` - Instructs the LLM to not exceed 1 sentence when generating the commit message.
 - `%{diff}` - The contents of the diff.
+- `%{branch_name}` - The name of the current branch. Available in "Commit Message" custom prompt only.
 
 ## Absorbing New Work