Clearly document unsupported/experimental/? status of 32-bit platforms

[slfritchie]

Howdy, all. While investigating WallarooLabs/wally#2309, I took a peek at the main README.md for this repo and didn't find any clear or obvious text that 32-bit platforms are experimental or unsupported or something else.

According to @dipinhora, 32-bit builds on some/all platforms don't pass the full test suite. It'd be good to say so in the README and any other appropriate documentation.

[dipinhora]

#1576 has some details for x86 builds.