docs: add note about ipfs.types.Buffer (#373)
docs: add note about ipfs.types.Buffer (#373)

I've spent an hour trying to figure out how to do `ipfs add` in the browser: docs describe the "Buffer" part really vaguely and don't mention that `ipfs.files.add` in fact creates a directory object unless used with a correctly created buffer. It works with an incorrectly created buffer, but creates a directory with auto-generated name:

```js
let data = new Uint8Array([0x41, 0x42, 0x43]);
let results = await ipfs.files.add(data);
```

Here `results` is

```json
[
  {
    "path": "QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn",
    "hash": "QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn",
    "size": 4
  }
]
```

Now open this hash in `ipfs.io` and try to figure out wtf that is and why size is 4. The `ipfs.types.Buffer` was the key.

ipfs/interface-ipfs-core