Update README.md

README.md

@@ -155,7 +155,7 @@ b) the slope of stress vs. stretch and
 c) the sign of the resulting stretch from a small superposed force in one direction.
 
 ## Hints and usage in FEM modules
-Please have a look at [casADi's documentation](https://web.casadi.org/). It is very powerful but unfortunately does not support all the Python stuff you would expect. For example Python's default if-else-statements can't be used in combination with symbolic conditions (use `math.if_else(cond, if_true, if_false)` instead).
+For tensor-valued material definitions use `MaterialTensor` (e.g. any stress-strain relation). Please have a look at [casADi's documentation](https://web.casadi.org/). It is very powerful but unfortunately does not support all the Python stuff you would expect. For example Python's default if-else-statements can't be used in combination with symbolic conditions (use `math.if_else(cond, if_true, if_false)` instead).
 
 Simple examples for using `matadi` with [`scikit-fem`](https://github.com/adtzlr/matadi/discussions/14#) as well as with [`felupe`](https://github.com/adtzlr/matadi/discussions/22) are shown in the Discussion section.