add cassandra2 (#6582)

weed/filer/cassandra2/README.txt

@@ -0,0 +1,15 @@
+1. create a keyspace
+
+CREATE KEYSPACE seaweedfs WITH replication = {'class':'SimpleStrategy', 'replication_factor' : 1};
+
+2. create filemeta table
+
+ USE seaweedfs;
+
+ CREATE TABLE filemeta (
+    dirhash bigint,
+    directory varchar,
+    name varchar,
+    meta blob,
+    PRIMARY KEY ((dirhash, directory), name)
+ ) WITH CLUSTERING ORDER BY (name ASC);