Update README to include other SLACK_COLOR valid values

README.md

@@ -48,7 +48,7 @@ SLACK_USERNAME       | `rtBot`                                               | C
 SLACK_MSG_AUTHOR     | `$GITHUB_ACTOR` (The person who triggered action).    | GitHub username of the person who has triggered the action. In case you want to modify it, please specify corrent GitHub username.
 SLACK_ICON           | ![rtBot Avatar](https://github.com/rtBot.png?size=32) | User/Bot icon shown with Slack message. It uses the URL supplied to this env variable to display the icon in slack message.
 SLACK_ICON_EMOJI     | -                                                     | User/Bot icon shown with Slack message, in case you do not wish to add a URL for slack icon as above, you can set slack emoji in this env variable. Example value: `:bell:` or any other valid slack emoji.
-SLACK_COLOR          | `good` (green)                                        | You can pass `${{ job.status }}` for automatic coloring or an RGB value like `#efefef` which would change color on left side vertical line of Slack message.
+SLACK_COLOR          | `good` (green)                                        | You can pass `${{ job.status }}` for automatic coloring or an RGB value like `#efefef` which would change color on left side vertical line of Slack message. Other valid values for this field are: `success`, `cancelled` or `failure`.
 SLACK_LINK_NAMES     | -                                                     | If set to `true`, enable mention in Slack message. 
 SLACK_MESSAGE        | Generated from git commit message.                    | The main Slack message in attachment. It is advised not to override this.
 SLACK_TITLE          | Message                                               | Title to use before main Slack message.
@@ -61,6 +61,7 @@ SLACK_ON_FAILURE     | -                                                     | I
 SLACK_ON_CANCEL      | -                                                     | If set, will send the provided message instead of the default message when the passed status (through ``SLACK_COLOR``) is `cancelled`.
 SLACK_CUSTOM_PAYLOAD | -                                                     | If you want to send a custom payload to slack, you can pass it as a string to this variable. This will override all other variables and send the custom payload to slack. Example: `SLACK_CUSTOM_PAYLOAD: '{"text": "Hello, World!"}'`, Note: This payload should be in JSON format, and is not validated by the action.
 
+
 You can see the action block with all variables as below:
 
 ```yml