Add mailing list and move screenshot at the end of README (#3872)

* README: add the mailing list

And mark the google group as deprecated

* README: move the screenshots at the end

So hopefully it'll be easier to find the resources

README.md

@@ -127,33 +127,14 @@ Installation & Configuration
 [See in the documentation](https://superset.incubator.apache.org/installation.html)
 
 
-More screenshots
-----------------
-
-![superset-security-menu](https://cloud.githubusercontent.com/assets/130878/20234707/0f565886-a835-11e6-9277-b4f5f4aa2fcc.png)
-
-![superset-slice-bubble](https://cloud.githubusercontent.com/assets/130878/20234708/0f57f3d0-a835-11e6-8268-fcefe8f868c8.png)
-
-![superset-slice-map](https://cloud.githubusercontent.com/assets/130878/20234709/0f5a5a44-a835-11e6-987a-1b6f8ac9922b.png)
-
-![superset-slice-multiline](https://cloud.githubusercontent.com/assets/130878/20234710/0f632d68-a835-11e6-98d1-542dcb618193.png)
-
-![superset-slice-sankey](https://cloud.githubusercontent.com/assets/130878/20234711/0f639136-a835-11e6-8721-fe5e48dab8e7.png)
-
-![superset-slice-view](https://cloud.githubusercontent.com/assets/130878/20234712/0f63c4c6-a835-11e6-8595-6091a6428fa9.png)
-
-![superset-sql-lab-2](https://cloud.githubusercontent.com/assets/130878/20234713/0f67b856-a835-11e6-9d50-7a52168f66fd.png)
-
-![superset-sql-lab](https://cloud.githubusercontent.com/assets/130878/20234714/0f68f45a-a835-11e6-9467-f47ad0af7e79.png)
-
-
 Resources
 -------------
-* [Superset Google Group](https://groups.google.com/forum/#!forum/airbnb_superset)
+* [Mailing list](https://lists.apache.org/list.html?dev@superset.apache.org/)
 * [Gitter (live chat) Channel](https://gitter.im/airbnb/superset)
 * [Docker image](https://hub.docker.com/r/amancevice/superset/) (community contributed)
 * [Slides from Strata (March 2016)](https://drive.google.com/open?id=0B5PVE0gzO81oOVJkdF9aNkJMSmM)
 * [Stackoverflow tag](https://stackoverflow.com/questions/tagged/apache-superset)
+* [DEPRECATED Google Group](https://groups.google.com/forum/#!forum/airbnb_superset)
 
 
 Contributing
@@ -197,3 +178,22 @@ the world know they are using Superset. Join our growing community!
  - [Yahoo!](https://yahoo.com/)
  - [Zalando](https://www.zalando.com)
 
+
+More screenshots
+----------------
+
+![superset-security-menu](https://cloud.githubusercontent.com/assets/130878/20234707/0f565886-a835-11e6-9277-b4f5f4aa2fcc.png)
+
+![superset-slice-bubble](https://cloud.githubusercontent.com/assets/130878/20234708/0f57f3d0-a835-11e6-8268-fcefe8f868c8.png)
+
+![superset-slice-map](https://cloud.githubusercontent.com/assets/130878/20234709/0f5a5a44-a835-11e6-987a-1b6f8ac9922b.png)
+
+![superset-slice-multiline](https://cloud.githubusercontent.com/assets/130878/20234710/0f632d68-a835-11e6-98d1-542dcb618193.png)
+
+![superset-slice-sankey](https://cloud.githubusercontent.com/assets/130878/20234711/0f639136-a835-11e6-8721-fe5e48dab8e7.png)
+
+![superset-slice-view](https://cloud.githubusercontent.com/assets/130878/20234712/0f63c4c6-a835-11e6-8595-6091a6428fa9.png)
+
+![superset-sql-lab-2](https://cloud.githubusercontent.com/assets/130878/20234713/0f67b856-a835-11e6-9d50-7a52168f66fd.png)
+
+![superset-sql-lab](https://cloud.githubusercontent.com/assets/130878/20234714/0f68f45a-a835-11e6-9467-f47ad0af7e79.png)