docs: Add bs-fetch peer dependency to README

[cem2ran]

After teamwalnut#104 was merged, you have to explicitly install bs-fetch which may not be obvious to everyone until you try to compile and it fails:

File "bsconfig.json", line 1
Error: package bs-fetch not found or built 
- Did you install it?
- If you did, did you run `bsb -make-world`?
error Command failed with exit code 2.

So to avoid any friction in installing the library I think it's preferable to make this explicit in step 1 such that you are not met with the error when you try to compile.