Added TODO

TODO.md

+To-do:
+-----
+
+    * finish:
+        * mksheet.pl
+        * htlogr: implement severities
+        * htlogr: straighten up parameters
+
+    * document:
+        * usage of mksheet.pl
+
+    * test:
+        * htlogr->data()
+        * htlogr->data() and mksheet.pl
+