Cover Sentry's milksnake on the extension modules page

[ncoghlan]

Updated to refer to https://github.com/getsentry/milksnake instead

======== Original text: @mitsuhiko's https://github.com/mitsuhiko/snaek is an elegant approach to writing & distributing Rust-accelerated extension modules that minimises the number of binary wheels you need to create by relying on cffi to manage the integration with the Python runtime.