Serve raw files through RawGit instead of having people download raw "addons_installer.FCMacro"

[luzpaz]

Right now in order for people to use the addon-macro they need to (per https://github.com/FreeCAD/FreeCAD-addons#1-using-the-installer-macro)

Click on addons_installer.FCMacro,right-click on the Raw button at the top of the page and choose Save as...

Enter RawGit

RawGit serves raw files directly from GitHub with proper Content-Type headers.

I've taken the liberty to add the addons_installer.FCMacro link to RawGit
https://github.com/FreeCAD/FreeCAD-addons/blob/master/addons_installer.FCMacro

Here is the generated production link:
https://cdn.rawgit.com/FreeCAD/FreeCAD-addons/cf4cfbaf/addons_installer.FCMacro

Here is the generated development link:
https://rawgit.com/FreeCAD/FreeCAD-addons/master/addons_installer.FCMacro

So we can essentially add one of these links to the instructions: https://github.com/FreeCAD/FreeCAD-addons#1-using-the-installer-macro