Flesh out README

README.md

@@ -6,4 +6,94 @@ Rust Tree-sitter
 
 Rust bindings to the [Tree-sitter][] parsing library.
 
+### Basic Usage
+
+First, create a parser:
+
+```rust
+let parser = Parser::new();
+```
+
+Then assign a language to the parser. Tree-sitter languages consist of generated C code. To use them from rust, you must declare them as `extern "C"` functions and invoke them with `unsafe`:
+
+```rust
+extern "C" fn tree_sitter_c() -> Language;
+extern "C" fn tree_sitter_rust() -> Language;
+extern "C" fn tree_sitter_javascript() -> Language;
+
+parser.set_language(unsafe { tree_sitter_rust() }).unwrap();
+```
+
+Now you can parse source code:
+
+```rust
+let source_code = "fn test() {}";
+
+let tree = parser.parse_str(source_code, None);
+let root_node = tree.root_node();
+assert_eq!(root_node.kind(), "source_file");
+assert_eq!(root_node.start_position().column, 0);
+assert_eq!(root_node.end_position().column, 12);
+```
+
+### Editing
+
+Once you have a syntax tree, you can update it when your source code changes:
+
+```rust
+let new_source_code = "fn test(a: u32) {}"
+
+tree.edit(InputEdit {
+  start_byte: 8,
+  old_end_byte: 8,
+  new_end_byte: 14,
+  start_position: Point::new(0, 8),
+  old_end_position: Point::new(0, 8),
+  new_end_position: Point::new(0, 14),
+});
+let new_tree = parser.parse_str(new_source_code, Some(tree));
+```
+
+### Text Input
+
+
+The code can be provided either as a simple string or by any type that implements Tree-sitter's `Utf8Input` or `Utf16Input` traits:
+
+```rust
+struct LineWiseInput {
+    lines: &'static [&'static str],
+    row: usize,
+    column: usize,
+}
+
+impl tree_sitter::Utf8Input for LineWiseInput {
+    fn read(&mut self) -> &[u8] {
+        if self.row < self.lines.len() {
+            let result = &self.lines[self.row].as_bytes()[self.column..];
+            self.row += 1;
+            self.column = 0;
+            result
+        } else {
+            &[]
+        }
+    }
+
+    fn seek(&mut self, _byte: u32, position: Point) {
+        self.row = position.row as usize;
+        self.column = position.column as usize;
+    }
+}
+
+let mut input = LineBasedInput {
+    lines: &[
+        "pub fn main() {",
+        "}",
+    ],
+    row: 0,
+    column: 0
+};
+
+let tree = parser.parse_utf8(&mut input, None).unwrap();
+```
+
 [tree-sitter]: https://github.com/tree-sitter/tree-sitter