Document that "features should be additive"

[kennytm]

Although it seems to be well-known, I find it surprising that the requirement "features should be additive" is not documented at all on docs.crates.io. Disrespecting this leads to tricky errors when merging features like chyh1990/yaml-rust#44 and rust-lang/cargo#4323 (= rust-bakery/nom#544).

rust-lang/rfcs#1841 mentioned some documentation changes which should better be added here.

[kennytm]

Looks like doc.crates.io is a GitHub page which is part of cargo, not crates.io. I'll file the issue there instead. Sorry for disturbance.