Remove README include from normal build

[TerryRPatterson]

I am messing around with packaging ariadne as a rpm.
By default README files are marked as doc files which are
often excluded from installation in automated installs.
This results in compilation of lib.rs failing as it can't include
README.

Would it be possible to make the include_str conditional?
Something like #![cfg_attr(doc, doc = include_str!("../README.md"))]

I tried to write a small PR for this but I ended up running into the #![deny(missing_docs)].
The lint could be addressed with another doc macro like:
#![cfg_attr(not(doc), doc="Module docs excluded from non doc build")]
but I am not sure if y'all would be okay with that change.

Would y'all be okay with a change like that?