expand readme a bit

README.md

@@ -4,6 +4,17 @@ This is an IPLD ADL that provides string based pathing for protobuf nodes. The t
 
 Note that while it works internally with go-codec-dagpb, the Reify method (used to get a UnixFSNode from a DagPB node should actually work successfully with go-ipld-prime-proto nodes)
 
+## Usage
+
+The primary interaction with this package is to register an ADL on a link system. This is done with via a helper method.
+
+```go
+AddUnixFSReificationToLinkSystem(lsys *ipld.LinkSystem)
+```
+
+For link systems which have UnixFS reification registered, two ADLs will be available to the [`InterpretAs`](https://ipld.io/specs/selectors/) selector: 'unixfs' and 'unixfs-preload'. The different between these two ADLs is that the preload variant will access all blocks within a UnixFS Object (file or directory) when that object is accessed by a selector traversal. The non-preload variant in contrast will only access the subset of blocks strictly needed for the traversal. In practice, this means the subset of a sharded directory needed to access a specific file, or the sub-range of a file directly accessed by a range selector.
+
+
 ## License
 
 Apache-2.0/MIT © Protocol Labs