Add API UseCase.

UseCases.mdwn

@@ -17,3 +17,4 @@ You should add your use case as a subpage of the [[UseCases]] page, to create a
 - [[Collaborating]]
 - [[Fund Accounting|UseCases/FundAccounting]]
 - [[Double-entry Accounting|UseCases/DoubleEntryAccounting]]
+- [[API|UseCases/API]]

UseCases/API.mdwn

@@ -0,0 +1,13 @@
+# API
+
+As a developer, it's essential that I have access to a clearly documented API
+(at least) to read and (possibly) to write/update the financial data.
+
+Specifically, the need to generate reports and analyze data in new ways is
+essential, and while many systems try to give some sort of "report builder",
+which are perhaps useful for some needs, are never adequate to foresee every
+type of necessary issue.
+
+The API should allow for not only custom reports, but the ability to generate
+a programmatic analysis.  For example, this sort of API would be used to
+compute the [[public support test for USA 501(c)(3) organizations|UseCases/PublicSupportTest]].