Updated README with test info

Also, added some info pointing to the wiki documentation/conversation.

Change-Id: I54992a2e4140b10df565b8f56fd3d40655c43d66

README.md

 edx-analytics-data-api
 ======================
+
+See https://edx-wiki.atlassian.net/wiki/display/AN/Analytics+Data+API for more details.
+
+Running Tests
+-------------
+
+Run `make validate`.