Agile - INVEST
敏捷项目思想：
I - Independent
PBI (Product Backlog Item) should be self-contained, in a way that there is no inherent dependency on anthoer PBI.
N - Negotiable
PBIs are not explicit contracts and should leave space for discussion.
V - Valuable
A PBI must deliver to the stakeholders.
E - Estimable
You must always be able to estimate the size of a PBI.
S - Small
PBIs should not be so big as to become impossible to plain.
T - Testable
The PBI or its related description must provide the necessary information to make test development possible.