wasm/README.md: Add a note about the Ruby built for wasm. [ci skip]

The Ruby built for wasm cannot be execute without a WebAssembly runtime.

```
$ ruby-wasm32-wasi/usr/local/bin/ruby -e 'puts "a"'
bash: ruby-wasm32-wasi/usr/local/bin/ruby: cannot execute binary file: Exec format error
```

Because the Ruby's file type is different from the one built normally, that is
the `/usr/local/ruby-3.2.0-preview2/bin/ruby` below.

```
$ file ruby-wasm32-wasi/usr/local/bin/ruby
ruby-wasm32-wasi/usr/local/bin/ruby: WebAssembly (wasm) binary module version 0x1 (MVP)

$ file /usr/local/ruby-3.2.0-preview2/bin/ruby
/usr/local/ruby-3.2.0-preview2/bin/ruby: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=a37822085e285c0971159982e7642dda88cea606, for GNU/Linux 3.2.0, with debug_info, not stripped
```

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