add existing documentation

doc/abi.txt

@@ -0,0 +1,21 @@
+=== TASK ENVIRONMENT ====
+
+This data is placed <somewhere> in the address space
+of a new task.
+
+environment_t {
+	/** array of string pointers for command line arguments. */
+	size_t argc;
+	char **argv;
+
+	/** handles to the system manager and session manager services
+	    respectively */
+	kern_handle_t system;
+	kern_handle_t session;
+
+	/** handles to the pagebuf objects storing the initial thread
+	    stack and task heap respectively */
+	kern_handle_t stack;
+	kern_handle_t heap;
+}
+