Adds some basic documentation about creating new ABCv6 models

doc/abc-v6-export-basics.md

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+The absolute minimum your Blender scene must have to export successfully to ABCv6 is:
+- One Armature
+- One Mesh with an active UV Map
+- One Action with at least 1 Keyframe
+
+An example .blend can be found [here](./abc-v6-export-basics/minimal_abc-v6_no_anim.blend)
+
+![](./abc-v6-export-basics/minimum_scene.png)
+
+## Making a basic ABCv6 model
+First make an armature, and in object mode set the X rotation to 90 degrees, and the X and Z scale to -1.
+
+![](./abc-v6-export-basics/armature_transform.png)
+
+Next, make a mesh, and set its parent to the armature you just made. Then add an armature modifier for the same armature.
+
+![](./abc-v6-export-basics/mesh_armature_modifier.png)
+
+For every bone in your armature, the mesh *must* have a corresponding vertex group with the same name. Assign vertices you want to be affected by the bone here with a weight 1. *Never* assign multiple nodes to the same vertex, that is not supported by the format.
+
+Finally create an action. Actions start at frame 0, and must have at least 1 keyframe. Actions named with a "d_" prefix will not be exported, it is expected these contain vertex animation keyframes.
+
+Each bone must have the same amount of keyframes. They should be at the same times as the root node's **W Quaternion Rotation lane**. Only **location** and **rotation_quaternion** (other rotation types are not currently supported) lanes will be exported, scaling is not supported by the format.
+
+![](./abc-v6-export-basics/action_editor.png)
+
+## Exporting a basic ABCv6 model
+Finally we can export our model. First we need to make sure the mesh is *triangulated* (ctrl + T).
+
+Then we wind the triangles the correct way by *calculating the inside* (ctrl + shift + N).
+
+Now, go to File -> Export -> Lithtech ABC, select the armature you want to export (should be done automatically), and choose the version: ABC v6 (Lithtech 1.0), and click export.
+
+# Issues you may encounter
+- If some of your bones try to converge at [0, 0, 0] after export, it means they have fewer keyframes than the root node.
+- If you think your exported animation is missing some keyframes, check that you've set a rotation keyframe on your root node at the expected time.
+- If the bounding boxes are too small or too large, make sure you aren't scaling any of your objects.
+- If the export fails with an "active UV index 0 out of range" error, make sure you're not in mesh edit mode, select the mesh object, and click on the UV map you want to export, and try again.
+
+![](./abc-v6-export-basics/uv_error.png)
+
+## Final product
+I named it BERETTA_PU.ABC, and put it in BLOOD2/MODELS/POWERUPS to make it appear in 3rd person and when dropped on the ground:
+
+![](./abc-v6-export-basics/replaced_beretta_pickup.png)
+
+## Misc. Blender
+- There's no way to export a frame command string from Blender yet, so you'll have to put it in ModelEdit after export to do this. Make sure to press enter after typing to confirm the string.
+- If making a first person weapon model it may be helpful to create a camera object. I've found a Z position of 1.2m, and a rotation of X 90 degrees, and Z 180 degrees, with a 100 degree FOV to look fairly close to the game (in 16:9, with widescreen patches).