message: Add Gentoo dependencies to CONTRIBUTING.md and mention qtchooser
diff: @@ -124,6 +124,16 @@ For Archlinux (as of 2016.10.03) you'll need:
  sudo pacman -S libx11 libxcb xcb-util-keysyms mesa libgl qt5-base qt5-x11extras cmake
  ```
  +For Gentoo (as of 2017.02.24), you'll need:
  +
  +```
  +sudo emerge --ask x11-libs/libX11 x11-libs/libxcb x11-libs/xcb-util-keysyms dev-util/cmake dev-qt/qtcore dev-qt/qtgui dev-qt/qtwidgets dev-qt/qtsvg dev-qt/qtx11extras
  +```
  +
  +Checking that at least Qt 5.6 installs.
  +
  +On any distribution if you find qmake isn't available under its default name, or if `qmake -v` lists a Qt4 version, make sure you have qtchooser installed in your package manager and use it to select Qt5. This might be done by exporting `QT_SELECT=qt5`, but check with your distribution for details.
  +
  If you know the required packages for another distribution, please share (or pull request this file!)
  ### Mac
  