RFC: document compatibility properties of serialize

[JeffBezanson]

For every 1.x release so far, we have kept serialize backwards compatible for reading. Since this has been possible for 7 releases, the time has probably come to officially promise it. While the "syntax" of the data remains readable, there are still some caveats I have tried to point out. Did I miss anything?

[DhairyaLGandhi]

This is great! Thank you! One thought I had was that in ML it is fairly common to specify lots of small details as anonymous functions and closures all over the place. Serializing those and reading them back in (with the intention to train further) is also pretty crucial since it can be quite difficult to train some of these models over and over again. We have used BSON.jl so far, and while that has its quirks, it has been a good tool in the arsenal. We also have plans on a "data only" storage for optional cases, but I guess that is somewhat separate.

What do you think would be a good way of maintaining compatibility with serialized anonymous functions?

[ViralBShah]

@JeffBezanson This would be great to merge.

[timholy]

#42016 changes a couple of tags, not sure whether that's incompatible (some of the common symbols had to be jettisoned to make space for new "real" tags).