Update README to include derivatives

Author: jishnub

README.md

@@ -1,6 +1,7 @@
 # LegendrePolynomials.jl
 
 ![CI](https://github.com/jishnub/LegendrePolynomials.jl/workflows/CI/badge.svg?branch=master)
+[![codecov](https://codecov.io/gh/jishnub/LegendrePolynomials.jl/branch/master/graph/badge.svg?token=PGMRgetYmz)](https://codecov.io/gh/jishnub/LegendrePolynomials.jl)
 [![stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://jishnub.github.io/LegendrePolynomials.jl/stable)
 [![dev](https://img.shields.io/badge/docs-latest-blue.svg)](https://jishnub.github.io/LegendrePolynomials.jl/dev)
 
@@ -25,7 +26,14 @@ julia> Pl(0.5, 20)
 -0.04835838106737356
 ```
 
-To compute all the polynomials for `0 <= l <= lmax`, use 
+To compute the n-th derivative of the Legendre polynomial of degree `l` at the argument `x`, use `dnPl(x, l, n)`:
+
+```julia
+julia> dnPl(0.5, 10, 5)
+-61760.91796875
+```
+
+To compute all the polynomials for `0 <= l <= lmax`, use `collectPl(x; lmax)`
 
 ```julia
 julia> collectPl(0.5, lmax = 5)
@@ -38,6 +46,19 @@ julia> collectPl(0.5, lmax = 5)
   0.08984375
 ```
 
+To compute all the n-th derivatives for `0 <= l <= lmax`, use `collectdnPl(x; n, lmax)`
+
+```julia
+julia> collectdnPl(0.5, lmax = 5, n = 3)
+6-element OffsetArray(::Array{Float64,1}, 0:5) with eltype Float64 with indices 0:5:
+  0.0
+  0.0
+  0.0
+ 15.0
+ 52.5
+ 65.625
+```
+
 Check the documentation for other usage.
 
 # License