add blog post about docs.rs changed default targets

[syphar]

coming from the promotion & demotion for the apple targets, we will change the list of default targets we build for crates, if they don't provide a list of targets.

See rust-lang/docs.rs#2926 and rust-lang/docs.rs#2927

Rendered

[programmerjake]

maybe delay so they can be announced together if we want rust-lang/docs.rs#2940 too?

[syphar]

maybe delay so they can be announced together if we want rust-lang/docs.rs#2940 too?

@programmerjake I think we'll do that too.

Can you give me some input how to formulate the reasoning the best way?