Add attachment to example code. Refs #62

Author: Rich Leland

examples/transmissions/a-file.txt

@@ -0,0 +1 @@
+Woo hoo! This attachment was added via python-sparkpost.

examples/transmissions/send_transmission.py

@@ -1,5 +1,10 @@
+import os
+
 from sparkpost import SparkPost
 
+parent_dir = os.path.dirname(os.path.realpath(__file__))
+attachment_path = os.path.abspath(os.path.join(parent_dir, "a-file.txt"))
+
 sp = SparkPost()
 
 response = sp.transmissions.send(
@@ -23,6 +28,13 @@
     },
     track_opens=True,
     track_clicks=True,
+    attachments=[
+        {
+            "name": "test.txt",
+            "type": "text/plain",
+            "filename": attachment_path
+        }
+    ],
     campaign='sdk example',
     metadata={
         'key': 'value',