build: add usage in readme

README.md

@@ -2,8 +2,23 @@
 
 Code IntelliSense for [Skia's shading language](https://skia.org/docs/user/sksl/).
 
+## Usage
+
+Install the [extension](https://marketplace.visualstudio.com/items?itemName=seven332.vscode-sksl).
+
+The kind of SkSL must be presented in source code.
+
+```
+// kind=shader
+half4 main(float2 coord) {
+    return float4(1, 1, 1, 1);
+}
+```
+
+It could be `shader`, `colorfilter`, `meshfrag` or `meshvert`.
+
 ## Features
 
-- [x] Document Symbol
-- [x] Document Formatting (may **NOT** work as expected)
-- [x] Semantic Token
+-   [x] Document Symbol
+-   [x] Document Formatting (may **NOT** work as expected)
+-   [x] Semantic Token