update usage section (#4)

README.md

@@ -1,12 +1,15 @@
 # Publish ruby gem action
 
-Action that builds a gem based on a gemspec and publish it to GitHub packages or Rubygems
+Action that builds a gem based on a gemspec and publishes to GitHub packages or Rubygems
 
 ## Usage
 
 ```yml
-steps:
- - name: Example
- id: example
- uses: actions/example@v1
+---
+- name: Publish gem to rubygems or gh packages
+ uses: juliocabrera/publish-ruby-gem-action@v1
+ with:
+ github_token: ${{secrets.GITHUB_TOKEN}} # optional
+ rubygems_api_key: ${{secrets.RUBYGEMS_API_KEY}} # optional
+ github_username: octocat # optional: default value is your username in github
 ```