Document coordinate systems in README

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

Remove trailing newlines from README

Host coordinate images locally

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)