Update book to clarify this is not what is on crates.io

[nikomatsakis]

In #501 we removed the caveat and link to the older book. We should include the old book as an appendix for historical reference and also add a caveat that this is an in-progress version of salsa available on git master but not crates.io.

[ofek]

Is there an ETA for publishing a new version (even RC)?

[aaronc]

It would be good to know when there will be any sort of tagged pre-release of what's in the book. I was pretty confused at first until I realized that the book reflects git main.