Merge pull request #1799 from petterreinholdtsen/hal-comp-ddt-man

Add description, note and author for the ddt HAL component.

Author: c-morley

src/hal/components/ddt.comp

@@ -15,6 +15,23 @@
 //   along with this program; if not, write to the Free Software
 //   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 component ddt "Compute the derivative of the input function";
+author "Jeff Epler";
+
+description """
+For every function call from the real time thread, calculate the
+difference between the old and current input value divided by the
+timer elapsed since the last call.
+""";
+
+notes """
+
+As this only work on two consecutive input values, it will only work
+well if the input change every function call, and not work so well if
+the rate of change is very low and the input change do not happen
+every time the real time function is called.
+
+""";
+
 
 pin in float in;
 pin out float out;