Auto merge of #3745 - hi-rustin:rustin-patch-docs, r=Turbo87
Auto merge of #3745 - hi-rustin:rustin-patch-docs, r=Turbo87

Better docs for send email in development

Might be helpful for new contributors!

[rendered](https://github.com/hi-rustin/crates.io/blob/rustin-patch-docs/docs/CONTRIBUTING.md#using-mailgun-to-send-emails)

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io