Update README.md

README.md

@@ -26,4 +26,6 @@ c[(2, 3, 4, 5)]
 ```
 This outputs a dictionary mapping the set of sites `i, j, k, l` to the resulting correlator `<Sz_i Sz_j Sz_k Sz_l>`, so in the last line above we are accessing the result for the correlator `<Sz_2 Sz_3 Sz_4 Sz_5>`.
 
+## Known issues
+
 Note that currently, when fermionic sites are being used, this package does not handle the case of mixed `A`- and `C`-operators acting on the same sites properly (i.e. it doesn't handle inputs `(2, 3, 3, 5)` properly for `("Adagdn", "Adagup", "Cup", "Cdn")`, say compared to how that operator would be constructed in the `OpSum` to `MPO` converter in ITensorMPS.jl). This is not very common, and generally users should just use `C`-operators if they want to compute correlators involving fermionic creation and annhilation operators. When `C`-operators are used with fermionic sites the `correlator` function will automatically insert the correct Jordan-Wigner strings to handle fermionic statistics.