add doc for hierarchy storage
Merge pull request #543 from shimingfei/AddDocForHierarchystore

add doc for hierarchy storage
update hierarchical storage doc
tweak document
modify doc for hierarchy storage
Merge pull request #544 from shimingfei/AddDocForHierarchystore

modify doc for hierarchy storage
tweak hierarchical storage doc
modify and add more description
Merge pull request #545 from shimingfei/AddDocForHierarchystore

modify and add more description
tweak doc for hstore
update beginner task's url