docs: update readme mentioning env vars

README.md

@@ -103,6 +103,21 @@ https://discord.com/developers/docs/resources/webhook#execute-webhook.**
 - `content` (**required**) (_string_): The message contents (up to 2000
   characters). For formatting your content see
   [Markdown Text 101](https://support.discord.com/hc/en-us/articles/210298617-Markdown-Text-101-Chat-Formatting-Bold-Italic-Underline).
+
+You can add the following parameters in your content that will be replaced with their respective ConcourseCI values:
+
+- ATC_EXTERNAL_URL - The external URL to your ConcourseCI instance
+- BUILD_ID - The ID of the build
+- BUILD_JOB_ID - The ID of the job
+- BUILD_JOB_NAME - The name of the job
+- BUILD_NAME - The name of the build
+- BUILD_PIPELINE_ID - The ID of the pipeline
+- BUILD_PIPELINE_NAME - The name of the pipeline
+- BUILD_TEAM_ID - The ID of the team
+- BUILD_TEAM_NAME - The name of the team
+
+To insert these values, use the following syntax: `${BUILD_ID}` or `$BUILD_ID`
+
 - `username` (_string_): Override the default username of the webhook
 - `avatar_url` (_string)_: Any text wanted to ultimately appear on the page as
   the title of the message.