Mention that correlation matrices are not yet supported

Fix #41.

README.md

@@ -69,16 +69,16 @@ easy-to-use calculator.
   automatic differentiation, and much more.
 * Integration with [`Plots.jl`](https://github.com/JuliaPlots/Plots.jl).
 
-Further features are expected to come in the future, see the section "How Can I
-Help?" and the TODO list below.
-
 The method used to handle functional correlation is described in this paper:
 
 * M. Giordano, 2016, "Uncertainty propagation with functionally correlated
   quantities", [arXiv:1610.08716](http://arxiv.org/abs/1610.08716)
   (Bibcode:
   [`2016arXiv161008716G`](http://adsabs.harvard.edu/abs/2016arXiv161008716G))
 
+A current limitation of the package is that it is not yet possible to define
+quantities related by a correlation matrix.
+
 If you use use this package for your research, please cite it.
 
 ### Documentation ###

docs/src/index.md

@@ -67,6 +67,9 @@ The main features of the package are:
   differentiation, and much more.
 - Integration with [Plots.jl](https://github.com/JuliaPlots/Plots.jl).
 
+A current limitation of the package is that it is not yet possible to define
+quantities related by a correlation matrix.
+
 ## Citation and license
 
 The method used to handle functional correlation is described in this paper:
@@ -81,7 +84,7 @@ convience, a BibTeX entry is provided in the
 file.
 
 Other features are expected to come in the future, see the [How Can I
-Help?](@ref) section.
+Help?](@ref) section and the [TODO](@ref) list.
 
 The `Measurements.jl` package is licensed under the MIT "Expat" License. The
 original author is Mosè Giordano.

docs/src/todo.md

@@ -5,6 +5,12 @@ The package is developed at
 https://github.com/JuliaPhysics/Measurements.jl. There you can submit bug
 reports, make suggestions, and propose pull requests.
 
+TODO
+----
+
+* Allow defining quantities related by a correlation matrix and correctly
+  propagate uncertainty in this case
+
 How Can I Help?
 ---------------