todo to readme

Author: franckgaga

README.md

@@ -126,7 +126,9 @@ for more detailed examples.
   - [x] manipulated inputs
   - [x] measured outputs
 - [x] bumpless manual to automatic transfer for control with a proper initial estimate
-- [x] observers in predictor form to ease control applications
+- [ ] estimators in two forms:
+  - [x] predictor (or delayed) form to reduce computational load
+  - [ ] filter (or current) form to improve accuracy and robustness
 - [x] moving horizon estimator in two formulations:
   - [x] linear plant models (quadratic optimization)
   - [x] nonlinear plant models (nonlinear optimization)