docs: update readme

.github/main.workflow

@@ -9,5 +9,5 @@ action "Post message to Telegram" {
     "TELEGRAM_TOKEN",
     "TELEGRAM_TO",
   ]
-  args = "Hello World!"
+  args = "A new commit has been pushed."
 }

README.md

@@ -13,6 +13,52 @@ action "Post message to Telegram" {
     "TELEGRAM_TOKEN",
     "TELEGRAM_TO",
   ]
-  args = "Hello World"
+  args = "A new commit has been pushed."
+}
+```
+
+## Secrets
+
+Geting started with [Telegram Bot API](https://core.telegram.org/bots/api).
+
+* `TELEGRAM_TOKEN`: Telegram authorization token.
+* `TELEGRAM_TO`: Unique identifier for this chat.
+
+How to get unique identifier from telegram api:
+
+```
+$ curl https://api.telegram.org/bot<token>/getUpdates
+```
+
+See the result: (get chat id like `65382999`)
+
+```json
+{
+  "ok": true,
+  "result": [
+    {
+      "update_id": 664568113,
+      "message": {
+        "message_id": 8423,
+        "from": {
+          "id": 65382999,
+          "is_bot": false,
+          "first_name": "Bo-Yi",
+          "last_name": "Wu (appleboy)",
+          "username": "appleboy46",
+          "language_code": "en"
+        },
+        "chat": {
+          "id": 65382999,
+          "first_name": "Bo-Yi",
+          "last_name": "Wu (appleboy)",
+          "username": "appleboy46",
+          "type": "private"
+        },
+        "date": 1550333434,
+        "text": "?"
+      }
+    }
+  ]
 }
 ```