updated get message api (#75)

* updated get message api

* Apply suggestions from code review

---------

Co-authored-by: Will Power <[email protected]>

Author: Som23Git

content/4.developer/1.api.md

@@ -52,3 +52,28 @@ For both these methods, the API will return the same information as the result.
   }
 }
 ```
+
+## GET Message
+
+To retrieve a message and its contents, use the `GET` method with the `id` (received when sending the message) and `_expansions` parameters (if you need more information than the basics) for the message from Postal. For more details, refer to the [Postal API documentation](https://postalserver.github.io/postal-api/controllers/messages/message.html).
+
+```json
+{
+  "id": 14,
+  "_expansions": true
+}
+```
+
+### Example cURL Request
+
+You can use the following cURL command to make the request:
+
+```bash
+curl --location 'https://<your.postal.domain.com>/api/v1/messages/message' \
+--header 'X-Server-API-Key: $yourAPIKeyFromTheCredentialsPage' \
+--header 'Content-Type: application/json' \
+--data '{
+  "id": 14,
+  "_expansions": true
+}'
+```