Docs: Added report API

doc/reports.txt

@@ -48,6 +48,33 @@ Here are some provisional examples of report commands:
   $ timew report week [±<tag> ...]
   $ timew report month|quarter|year
 
+
+Report API
+----------
+The report API is simple - it invokes a program and feeds it input. All output
+is generated by the program. When running this command:
+
+  $ timew report x ...
+
+Timewarrior then runs this:
+
+  $ ~/.timewarrior/extensions/x.*
+
+With standard input consisting of:
+
+  filter: ...
+  name1: value1
+
+  [
+  { ... },
+  { ... },
+  ...
+  ]
+
+That is, the input it line-oriented, with first a header block of name/value
+pairs, then a blank line, then the JSON filtered data. The header block has all
+configuration names flattened, i.e. no hierarchy.
+
 ---
 
 - The Taskwarrior hook script should use a set of attributes for time tracking