Document whether there is WASM support

[Bajix]

It's unclear whether WASM is supported. I assume it's probably not, but it would be informative if this was mentioned either way

[jdonszelmann]

It looks like there is not, at this moment (I tried to build the example test on wasm32-wasi and that did not work).

[mpapierski]

Is that even possible to implement linkme for wasm32 targets?

[dfoxfranke]

Bumping this issue. I noticed that inventory recently added WASM support, albeit only when targetting WASI or emscripten: dtolnay/inventory#71. I was wondering the same LLVM facilities could make it possible for linkme to support wasm32-unknown-unknown. (Since, unlike inventory, linkme doesn't depend on life-before-main, I suppose it shouldn't make any difference what OS functionality is available from the embedder?)