Update docs

Author: fkorotkov

README.md

@@ -2,10 +2,15 @@
 
 [![Build Status](https://api.cirrus-ci.com/github/cirrus-actions/rebase.svg)](https://cirrus-ci.com/github/cirrus-actions/rebase) [![](https://images.microbadger.com/badges/version/cirrusactions/rebase.svg)](https://microbadger.com/images/cirrusactions/rebase) [![](https://images.microbadger.com/badges/image/cirrusactions/rebase.svg)](https://microbadger.com/images/cirrusactions/rebase)
 
-After installation simply comment `/rebase` to trigger the action:
+After [installation](#installation) simply comment `/rebase` to trigger the action:
 
 ![rebase-action](https://user-images.githubusercontent.com/989066/51547853-14a57b00-1e35-11e9-841d-33114f0f0bd5.gif)
 
+# Supported Comments
+
+* `/rebase` - to perform the default rebase.
+* `/rebase --autosquash` - to perform a rebase with `--autosquash` argument.
+
 # Installation
 
 To configure the action simply add the following lines to your `.github/workflows/rebase.yml` workflow file: