Clarify preload behavior

Preload is confusing because it happens on both add and get, and because it isn't documented anywhere in depth. This clarifies what happens a bit. We should also write out the behavior in more detail like with circuit relay above (possibly in the form of a tutorial for setting up your own preload node)

see also ipfs#1459 (comment)

README.md

@@ -290,7 +290,7 @@ Configure circuit relay (see the [circuit relay tutorial](https://github.com/ipf
 |------|---------|
 | object | `{ enabled: true, addresses: [...] }` |
 
-Configure external nodes that will preload content added to this node.
+Configure remote preload nodes. The remote will preload content added on this node, and also attempt to preload objects requested by this node.
 
 - `enabled` (boolean): Enable content preloading (Default: `true`)
 - `addresses` (array): Multiaddr API addresses of nodes that should preload content. **NOTE:** nodes specified here should also be added to your node's bootstrap address list at [`config.Boostrap`](#optionsconfig).