[WIP][DOC] DML to R translation guide (#1007)
[WIP][DOC] DML to R translation guide (#1007)

* some common differences between R and DML
* The examples should be runnable to make the
  guide more interesting.
apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml