README.md: Mark ls as optional

mxinden · mxinden · commit 34bad788b5eb · 2022-09-22T16:34:53.000+01:00
go-multistream removed `ls` support in multiformats/go-multistream#76. This breaks compatibility with the multistream implementation in Rust see libp2p/rust-libp2p#2925. This commit updates the specification after the fact, explicitly making `ls` support optional.
diff --git a/README.md b/README.md
@@ -139,6 +139,8 @@ For example
 < ...
 ```
 
+Note that `ls` support is OPTIONAL, i.e. implementations MAY support sending and/or receiving `ls`. This in turn implies that implementations MUST NOT depend on a remote node supporting `ls`.
+
 #### Implementation Recommendations
 
 **Protocol Selection**
