Add comment to README.md stating that it's auto-generated and shouldn't be edited directly

[joshlf]

Update generate-readme.sh to emit a comment following the copyright comment that explains that:

• README.md is generated automatically
• Edits should be made in the doc comment on src/lib.rs or in generate-readme.sh

[varadtote]

Can assign ths task to me ?

[joshlf]

Done!