Remove readme field from C-METADATA

Author: taiki-e

src/checklist.md

@@ -35,7 +35,7 @@
   - [ ] Prose contains hyperlinks to relevant things ([C-LINK])
   - [ ] Cargo.toml includes all common metadata ([C-METADATA])
     - authors, description, license, homepage, documentation, repository,
-      readme, keywords, categories
+      keywords, categories
   - [ ] Crate sets html_root_url attribute "https://docs.rs/CRATE/X.Y.Z" ([C-HTML-ROOT])
   - [ ] Release notes document all significant changes ([C-RELNOTES])
   - [ ] Rustdoc does not show unhelpful implementation details ([C-HIDDEN])

src/documentation.md

@@ -187,7 +187,6 @@ values:
 - `description`
 - `license`
 - `repository`
-- `readme`
 - `keywords`
 - `categories`