Add README for poll plugin (#2089)

plugins/poll/README.md

@@ -0,0 +1,24 @@
+# Poll plugin
+
+Allows you to add a poll to the first post of a topic.
+
+# Usage
+
+1. Make your topic title start with "Poll: "
+2. Include a list in your post. The **first list** will be used.
+  * If you don't want it to be the first list, do this:
+
+   ```
+   Intro Text
+   
+    - Some list
+    - Some list con't
+   
+   Here are your choices:
+   
+   [poll]
+    - Option 1
+    - Option 2
+   [/poll]
+   ```
+