  Very recently a new family of layered materials, containing BiS2 planes was
discovered to be superconducting at temperatures up to Tc=10 K, raising
questions about the mechanism of superconductivity in these systems. Here, we
present state-of-the-art first principles calculations that directly address
this question and reveal several surprising findings. The parent compound
LaOBiS2 possesses anharmonic ferroelectric soft phonons at the zone center with
a rather large polarization of $\approx 10 \mu C/cm^2$, which is comparable to
the well-known ferroelectric BiFeO3. Upon electron doping, new unstable phonon
branches appear along the entire line Q=(q,q,0), causing Bi/S atoms to order in
a one-dimensional charge density wave (CDW). We find that BiS2 is a strong
electron-phonon coupled superconductor in the vicinity of competing
ferroelectric and CDW phases. Our results suggest new directions to tune the
balance between these phases and increase Tc in this new class of materials.
