Eigenvalues of symetric matrix and eigenvalues of symetric matrix plus rank one matrix

Question

If $M$ is a symetric matrix with eigenvalues $m_1 \geq m_2 ... \geq m_n$ is there any connection between those eigenvalues and the eigenvalues $m'_1 \geq m'_2 ... \geq m'_n$ of the matrix $M' = M + xx^T$ ? I think that it has to be that: $m'_i \geq m_{i+1} \leq m'_{i+1}$ ? Am I right? How it can be proofed?