Add guidance for structuring local file dependencies

I wanted to set up some local interface dependencies for wkg to fetch, and I could find the right way to structure the filesystem. With some trial/error and setting up the code, I did eventually find the right structure (this is the folder that I pointed to for `wasi.dev`):
```
drwxr-xr-x - brooks 29 Oct 10:40 ./
drwxr-xr-x - brooks 29 Oct 10:42 └── wasi/
drwxr-xr-x - brooks 29 Oct 10:45 ├── http/
.rw-r--r-- 38k brooks 29 Oct 10:45 │ ├── 0.2.0.wasm
drwxr-xr-x - brooks 29 Oct 10:44 ├── keyvalue/
.rw-r--r-- 13k brooks 29 Oct 10:43 │ ├── 0.2.0-draft.wasm
drwxr-xr-x - brooks 29 Oct 10:45 ├── logging/
.rw-r--r-- 1.4k brooks 29 Oct 10:45 │ ├── 0.1.0-draft.wasm
```

I think some guidance on the fact that files should be stored in the form of `//.wasm` would be great, or if I'm missing this please just let me know what I misinterpreted! Happy to contribute this to the README as well if that's desired, or an external docs site.