chore: update readme.md file with notice about the module name

[3imed-jaberi]

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.

[mreinstein]

This isn't a good decision. According to npm stats, koa-send is downloaded 800,000 times weekly. koa-send works fine, and now we're going to intentionally orphan everyone who doesn't update to @koa/send (hint: the people behind most of the software depending on this module). For what? so we can feel good about things being under a @koa namespace?