Expand readme and add screenshot

README.md

@@ -1,11 +1,16 @@
-# Difftastic
+# It's Difftastic!
 
-A language-aware word-level diff.
+Difftastic is an experimental structured diff tool that compares files
+based on their syntax.
 
-```
-$ cargo run before.js after.js
-$ cargo run --inline before.js after.js
-```
+![screenshot](img/difftastic.png)
+
+It is very much unfinished. It works reasonably on very parenthesised
+data (lisps, JSON), it works sometimes on other languages with
+sufficient parentheses (Rust, JS), and falls back to a line-oriented
+diff otherwise.
+
+See `config/syntax.toml` to see how languages are defined.
 
 ## Other Diff Techniques