Elaborate on UnitfulRecipes in readme (#456)

If I had known this would support plot axes with units, I would not have started down the road to #455.
Maybe this elaboration will help someone else.

[skip ci]

README.md

@@ -32,7 +32,7 @@ mathematical operations and collections that are found in Julia base.
 
 ### Feature additions
 
-- [UnitfulRecipes.jl](https://github.com/jw3126/UnitfulRecipes.jl): Adds automatic labels for [Plots.jl](https://github.com/JuliaPlots/Plots.jl).
+- [UnitfulRecipes.jl](https://github.com/jw3126/UnitfulRecipes.jl): Adds automatic labels for [Plots.jl](https://github.com/JuliaPlots/Plots.jl) and supports plot axes with units. 
 - [UnitfulIntegration.jl](https://github.com/PainterQubits/UnitfulIntegration.jl): Enables use of Unitful quantities with [QuadGK.jl](https://github.com/JuliaMath/QuadGK.jl). PRs for other integration packages are welcome.
 - [UnitfulEquivalences.jl](https://github.com/sostock/UnitfulEquivalences.jl): Enables conversion between equivalent quantities of different dimensions, e.g. between energy and wavelength of a photon.
 - [UnitfulLatexify.jl](https://github.com/gustaphe/UnitfulLatexify.jl): Pretty print units and quantities in LaTeX format.