Add discussion of a "opt-out" lint for "opt-in" traits

[nikomatsakis]

Per the concerns mentioned on PR #19 by @bill-myers and @cmr, one option to help library authors remember to implement Send and Share would be a lint that warns you about builtin traits that you could have implemented but did not. I envision this being part of a series of lints that enforce best practices for libraries (and I could even see having them warn by default in library mode, but perhaps not app mode). I intend to add discussion of this to the PR.

[ftxqxd]

Not really relevant any more since Copy is the only opt-in built-in trait, and that’s already got a lint.

[withoutboats]

As @P1start commented almost 2 years ago, this is no longer how Send and Sync work. Closing.