Clarify type of alphabetical order

[svetogam]

I don't know how important this really is, but while working on #349 I noticed that this list has gotten large enough that it sorts differently depending on the type of alphabetical order used. Maybe some of the ordering mistakes I found were due to this.

As I understand it, there are basically 3 types to consider: Word-by-word, letter-by-letter, and unicode ordering.

I propose to add in CONTRIBUTING.md a short blurb clarifying to order "letter-by-letter, ignoring everything but letters and numbers."

Unicode weirdness:

• godot-voxel
• GodotHook
• GodotSteam

Word-by-word weirdness:

• TileSet
• Tiled

Letter-by-letter weirdness:

• Godot Asset Library
• Godotes
• Godot Shaders