Update README

Author: Porges

README.md

@@ -2,14 +2,24 @@
 
 A Rust library & CLI for reading RDFa data from HTML.
 
-## Installation
+### Installation
 
 ```console
 $ cargo install html2rdf-cli
 ```
 
-## CLI usage
+### CLI usage
 
 ```console
 $ html2rdf http://games.porg.es/games/tic-tac-toe
 ```
+
+## Goals & Support
+
+Currently, html2rdf supports (most of) RDFa 1.1. It may support some of RDFa 1.0 but this has not been properly tested.
+
+Missing features are:
+- XML output is not canonicalized at all
+- (optional) RDF entailment is not performed
+
+Eventually, html2rdf will also implement Microdata and JSON-LD, but this has not been started at all.