Clarify in README that "node" should be an "Identifier" node

Thanks for the library.
It works with the latest version of acorn.

Currently, the README says:

```
declaration = source(node)

Given a node within an AST generated by esprima or acorn, determine the original declaration node in the tree.
```

After playing with the library, one thing I notice is that `node` should have `type` of `Identifier`. This should be clarified in README.

The example in README can be improved like this:

```js
...
var walk = require('acorn-walk')

...
walk.simple(ast, {
Identifier(node) {
var declNode = source(node)
...
}
});
```