New meta in LDConfig for default category icon in editors

[Travis Cobbs]

Here's a link to the draft spec.

I softened the language at bit. I also put the quite back around the filename since it's not really that big of a deal to me.

https://www.ldraw.org/avatar-spec.html

Some suggestions based on what is there now:

1. "allow editor" -> "allow an editor"
   
2. "For editor" -> "For an editor"
   
3. "or less) of the category" -> "or less) description of the category"
   
4. "9 digit rotation" -> "9 element rotation"
   

Note: a digit is a single character 0-9, so the rotation matrix cannot be 9 digit. I'm not sure "element" is the best word here, but think that it works well.