Update index.md

Author: dlfivefifty

docs/src/index.md

@@ -1,4 +1,13 @@
 # ClassicalOrthogonalPolynomials.jl
+A Julia package for classical orthogonal polynomials and expansions
 
-*Classical orthogonal polynomials in Julia*
+## Evaluation
 
+The simplest usage of this package is to evaluate classical
+orthogonal polynomials:
+```jldoctest
+julia> using ClassicalOrthogonalPolynomials
+
+julia> chebyshevt(5, 0.1) # T_5(0.1) == cos(5acos(0.1))
+0.48016
+```