Update Readme

Add using Expectations etc.
General syntax: expectation(x->f(x), Distribution(θ))

using Expectations, Distributions;
E=expectation;
const μ=0.0; const σ=1.0;
E(x->x, Normal(μ,σ))
E(x->exp(x), Normal(μ,σ))
exp(μ+ (σ^2)/2.)
E(x->x^2, Normal(μ,σ))
E(x->x^2+exp(x), Normal(μ,σ))
#
D = [Uniform(), Normal(), Gamma()];
d = MixtureModel(D);
E.(x->x, D)
E(x->x, d)     #expectation of a mixture
( E(x->x,D[1]) + E(x->x,D[2]) + E(x->x,D[3]) )/3
#
E(x->x^2, d) #expectation of a transformation of a mixture

# truncated dist
d=truncated(LogNormal(μ,σ),0.0,5.4)
E(x->x, d)
E(x->x^2, d)
# truncated mixture
d=truncated(MixtureModel(D),0.0,5.4)
E(x->x, d)
E(x->x^2, d)

If you can agree, I can submit PR.

Start with the repository README and review its existing usage examples and syntax. Add the using Expectations setup, expectation syntax, and the Julia distribution examples shown in the issue; done means the README clearly documents these examples.