feat: default GITHUB_TOKEN to github.token (#579)

* feat: default `GITHUB_TOKEN` to `github.token`

* Update README to clarify GITHUB_TOKEN usage

Clarify the usage of GITHUB_TOKEN in the README.

Author: MichaelDeBoey

README.md

@@ -8,7 +8,7 @@ Use this action in a workflow [triggered by a release](https://docs.github.com/e
 
 ## Inputs
 
-**GITHUB_TOKEN**
+**GITHUB_TOKEN** (optional)
 
 A GitHub access token with write access to your repo's issues, such as:
 

action.yml

@@ -8,7 +8,8 @@ inputs:
     description: |
       A GitHub personal access token with repo scope, such as
       secrets.GITHUB_TOKEN.
-    required: true
+    default: ${{ github.token }}
+    required: false
   comment-template:
     description: |
       Text template for the comment string.