Update libhdfs engine documention and options

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/examples/libhdfs.fio b/examples/libhdfs.fio
new file mode 100644
index 0000000..d5c0ba6
--- /dev/null
+++ b/examples/libhdfs.fio
@@ -0,0 +1,8 @@
+[global]
+runtime=300
+
+[hdfs]
+filename=dfs-perftest-base.dfs-perftest-base,9000
+ioengine=libhdfs
+rw=read
+bs=256k