docs: add missing refs API (#2642)

Author: Alan Shaw

README.md

@@ -774,6 +774,13 @@ The core API is grouped into several areas:
   - [`ipfs.files.stat(path, [options])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/FILES.md#filesstat)
   - [`ipfs.files.write(path, content, [options])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/FILES.md#fileswrite)
 
+- [refs](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/REFS.md)
+  - [`ipfs.refs(ipfsPath, [options])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/REFS.md#refs)
+  - [`ipfs.refsReadableStream(ipfsPath, [options])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/REFS.md#refsreadablestream)
+  - [`ipfs.refsPullStream(ipfsPath, [options])`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/REFS.md#refspullstream)
+  - [`ipfs.refs.local()`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/REFS.md#refslocal)
+  - [`ipfs.refs.localReadableStream()`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/REFS.md#refslocalreadablestream)
+  - [`ipfs.refs.localPullStream()`](https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/REFS.md#refslocalpullstream)
 
 #### Graph