Merge pull request #304 from mbta/cm/active-message-api-doc

doc: Active Message API doc

docs/tech_specs/pa_message_api.md

@@ -0,0 +1,54 @@
+# List Active Messages
+
+Lists all PA messages that are currently eligible to play.
+
+**URL** : `/api/active_pa_messages`
+
+**Method** : `GET`
+
+**Parameters**: None
+
+**API key required** : YES
+
+## Success Responses
+
+**Code** : `200 OK`
+
+**Response**:
+
+```json
+[
+  {
+    "id": 1,
+    "sign_ids": ["sign_1", "sign2"],
+    "priority": 0,
+    "interval_in_minutes": 4,
+    "visual_text": "This message will be played.",
+    "audio_text": "This message will be played."
+  },
+  {
+    "id": 2,
+    "sign_ids": ["sign_3", "sign4"],
+    "priority": 0,
+    "interval_in_minutes": 3,
+    "visual_text": "This message will be played.",
+    "audio_text": "This message will be played."
+  },
+  {
+    "id": 3,
+    "sign_ids": ["sign_1"],
+    "priority": 0,
+    "interval_in_minutes": 2,
+    "visual_text": "This message will be played.",
+    "audio_text": "This message will be played."
+  }
+]
+```
+
+## Failure Responses
+
+**Code** : `403 Forbidden`
+
+**Response**:
+
+`Invalid API key`