v0.3.1: Add CHANGELOG.md with the 0.3.1 release notes

Changed

• The scene and pause shortcuts are now Control-Option-1 to 9 and
  Control-Option-P instead of Option-only combinations, which stole ordinary
  characters on German and other keyboard layouts (#20).

Fixed

• Entering an invalid Key Light address no longer crashes the app; it shows an
  error instead (#22).
• Out-of-range numbers sent to the control interface no longer crash the app
  (#19).
• A light can be added manually even when others were already found, and the
  address field is usable and submits on Return (#25).
• Changing one Key Light setting no longer overwrites its other settings with
  stale values (#21).
• Selecting a scene applies that scene, not the one selected before it (#16).
• Changing scene, camera or quality while paused no longer switches the camera
  and lights back on (#17).
• The "stays sharp up to" zoom limit updates when the camera resolution
  changes (#18).
• A scene name being typed is kept when switching scenes without pressing
  Return (#23).
• Value fields in the inspector show the current scene's value after switching
  scenes, and an unfinished entry can no longer land in another scene.
• Semi-transparent overlays no longer brighten the picture sent to calls (#26).
• Colours are encoded in the correct video range, so saturated colours match
  between the preview and calls (#24).