KUDU-1945 Update docs with non-unique PK

Added small update to cover non-unique primary key. For further info, I
added a link to the examples folder. Right now we only have the C++
example in place for non-unique PK, I plan to translate that example to
Java and Python as well.

Change-Id: I84e1c6b85d4fdb5ac95bad611246c071a63bcd31
Reviewed-on: http://gerrit.cloudera.org:8080/19809
Tested-by: Kudu Jenkins
Reviewed-by: Wenzhe Zhou <wzhou@cloudera.com>
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
2 files changed