Update docs to make it clear that we support only the latest stable version of rust

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
DataFusion / Ballista can (and sometimes do) fail to compile or successfully execute all tests when run with a version of Rust other than the latest stable version.

This can cause confusion, most recently on #1040 -- #1040 (comment)

The docs do not appear to say anything about this policy
https://github.com/apache/arrow-datafusion/search?l=YAML&q=stable

Describe the solution you'd like
Add a note to the docs that says DataFusion / Ballista are only tested with the latest stable version of rust

Additional context
I added a similar note to arrow-rs not too long ago, that we might be able to crib from: apache/arrow-rs#726

[1nF0rmed]

@alamb if I'm not wrong, we would want to update the Datafusion Readme and the Ballista Readme with a note that says we only support the latest stable version of Rust.

Also, can I base the note on the PR you mentioned in Additional context?

[alamb]

Thanks @1nF0rmed !

@alamb if I'm not wrong, we would want to update the Datafusion Readme and the Ballista Readme with a note that says we only support the latest stable version of Rust.

Yes I think that is correct

Also, can I base the note on the PR you mentioned in Additional context?

Absolutely

[1nF0rmed]

@alamb If it's alright, I'd like to make a PR for this

[alamb]

Please do -- that would be great

[1nF0rmed]

Thank you for being soo responsive!
I have raised a PR for the same.

Would it be possible to add the hacktoberfest label to this issue?

[alamb]

Would it be possible to add the hacktoberfest label to this issue?

Sure -- this project doesn't have a hactoberfest label -- do I just create one like

https://hacktoberfest.digitalocean.com/

[1nF0rmed]

Yes, you can add a hacktoberfest label to this issue. It will get tracked then

[alamb]

Done