Correct Install Instructions

Reflecting on how the Qt 5 version of this is no longer maintained, I have changed the Linux install instructions accordingly.

Thanks in advance!
Merge pull request #339 from aborsim/patch-2

Correct Install Instructions
Autocontrast: use the exact portion of the viewer to compute.
Update INSTALL_OSX.md
Update INSTALL_OSX.md
Optional inputs are now displayed with a lower opacity than non
optional inputs.
Masks are displayed with dashes.
By default optional inputs (et masks) are hidden if the node is not
selected or hovered by the mouse, to avoid cluttering the UI.
This can be toggled on/off in the preferences (Nodegraph) or in the
right click menu.
Merge branch 'workshop' of github.com:MrKepzie/Natron into workshop
OfxEffectInstance: categorize new TuttleOFX plugins
Add dot, backdrop and diskcache icons
Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop
ColorKnob: don’t set alpha when choosing color with dialog since it can
only pick RGB anyway.
Merge branch 'workshop' of github.com:MrKepzie/Natron into workshop
Bug fix: viewer don’t connect A and B to the same node with SHIFT + 1
update packageOSX
Natron: use real IDs declared by plugins.
Merge branch 'workshop' of github.com:MrKepzie/Natron into workshop
Fix unit tets
Small bug fix
EffectInstance: small  bug fix, when identity use renderRoI and not
getImage
Fix typo