Add blit shader comment

[Davidster]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
Link to the issues addressed by this PR, or dependent PRs in other repositories

Description
This is just a little comment I added on this same shader when I copied it into my project, I figured it might be helpful for others to explain how the vertex shader works. I know that comments generally become outdated over time so I'd understand if this isn't wanted in the project.

Testing
Tested locally

[codecov-commenter]

Codecov Report

Merging #3301 (b3704e5) into master (0e4c7dd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3301   +/-   ##
=======================================
  Coverage   65.66%   65.66%           
=======================================
  Files          82       82           
  Lines       39490    39490           
=======================================
  Hits        25931    25931           
  Misses      13559    13559           

Impacted Files	Coverage Δ
wgpu-core/src/hub.rs	60.82% <0.00%> (ø)

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[cwfitzgerald]

Cool!