Change log for aern2-mp

• v 0.1.3.1 2018-11-21
  • small fixes, mainly documentation
• v 0.1.3.0 2018-11-20
  • only one MPFR backend - rounded
  • reduce backend-specific code
• v 0.1.2.0 2017-11-14
  • fix compilation with haskell-mpfr
• v 0.1.1.0 2017-11-14
  • using Claude Heiland-Allen's Numeric.Rounded.Simple
• v 0.1.0.1 2017-09-12
  • first release on Hackage
  • backends: hmpfr and (tweaked) rounded