Merge #1722

1722: Document coordinate systems in README r=cwfitzgerald a=MichaelMMacLeod

I had to do a bit of digging to find information about coordinate systems in
wgpu (asking on Matrix). This commit adds a table of coordinate systems to help
clarify the issue.

The table comes from the DX12 backend documentation of gfx-rs:
https://github.com/gfx-rs/gfx/tree/master/src/backend/dx12

**Connections**
(none)

**Description**
It was difficult for me to find information about what coordinate systems wgpu uses. This commit addresses this issue by adding documentation about wgpu's coordinate systems to the README.

**Testing**
This is purely a documentation change (I have written no tests).

If you want to see how the table looks on Github, check out my fork at https://github.com/MichaelMMacLeod/wgpu/tree/add-coords-to-readme


Co-authored-by: Michael MacLeod <michaelmmacleod@gmail.com>

README.md

@@ -61,3 +61,11 @@ cargo test -p wgpu -- <name-of-test>
 ```
 
 If you are a user and want a way to help contribute to wgpu, we always need more help writing test cases. 
+
+## Coordinate Systems
+
+wgpu uses the coordinate systems of DX and Metal:
+
+Render | Depth | Texture
+-------|-------|--------
+![render_coordinates](./etc/render_coordinates.png) | ![depth_coordinates](./etc/depth_coordinates.png) | ![texture_coordinates](./etc/texture_coordinates.png)