Add task execution documentation

Closes #17 (closed)

@valerii.kalashnikov @lyuben.penkovski

docs/task.md

@@ -36,6 +36,18 @@ given as input will be used as the body of the task request. The caller will rec
 immediately the `taskID` as response, and the result of the asynchronous task
 execution will be stored in the TSA Cache after the task is completed.
 
+The actual _Task execution_ is strictly bound to the _Task definition_. In order a _task_
+to be executed successfully, its _definition_ **must** contain either a `requestPolicy` OR
+`url` and `method`. When a `requestPolicy` is set in the _Task definition_, the task will
+evaluate it and will ignore the `url` and the `method`. If a `requestPolicy` is missing in
+the _Task definition_, the task will execute an HTTP request to the given `url` with the
+given `method`. If both `requestPolicy` AND `url` and `method` are missing in the _Task definition_,
+the task cannot be executed. Reference table:
+
+_Task definition_ contains: | `requestPolicy` only | `url` and `method` only | Both `requestPolicy` AND `url` and `method` | Neither
+--- | --- | --- | --- |---
+**_Task_ will execute** | `requestPolicy` | `url` and `method` | `requestPolicy` | None
+
 ### Task Executor Configuration
 
 There are two environment variables that control the level of concurrency