Add a note about the nightly the guide was written for
Add a note about the nightly the guide was written for

https://github.com/rust-lang/rust/pull/31487 included breaking changes
which cause codegen to no longer compile on the latest nightly. It will
likely be several days before I can synchronize everything to work on
both nightly and stable against the latest versions.

sgrif/diesel.rs-website,sgrif/diesel.rs-website,sgrif/diesel.rs-website