Riemann Sum Question

Suppose $f \in R(x)$ on $[0,1]$. Define $a_n = \frac{1}{n} \sum_{k=1}^n f\left(\frac{k}{n}\right)$ for all n. Prove $\{a_n\}_{n=1}^{\infty}$ converges to $\int_0^1 f ~dt$

I missed the class where they covered Riemann Sums. If I can get some help starting this off I would be grateful.