README tweak

Nobody has claimed this yet.

In the readme, you mention that the hypercore feed can be faked with:

> any append-only log that supports `.append()` and `.length`

But in testing, I found more was needed. So I made a simple minimal interface.

```js
// Fake hypercore feed for testing and learning
var values = []
var feed = {
append (value, cb) {
console.log('APPEND', {value, cb})
values.push(value)
nextTick(cb)
},
get (index, options, cb) {
console.log('GET', {index, options, cb})
nextTick(cb, null, values[index])
},
get length () {
console.log('LENGTH')
return values.length
},
ready (cb) {
console.log('READY', {cb})
nextTick(cb)
}
}
```

Start in the README at the statement describing the minimal hypercore feed interface, then compare it with the supplied fake feed example. Done means the README accurately documents the methods and property needed for testing; no test path is mentioned in the issue.

Written by the indexing model from the issue text.