add first version

Author: Miao (Wilson) Liu

README.md

@@ -1,2 +1,88 @@
 # Script.apex
-Evaluate Javascript expressions in Apex
+Script.apex help you run JavaScript code in Apex. Basically, it compiles JavaScript code into semantic nodes, which are then evaluated in native Apex. The JavaScript expression parser in Script.apex is porting from [jsep](http://jsep.from.so/).
+
+## Features
+- Parse JavaScript code into semantic nodes
+- Evaluate nodes in Apex
+- Parsed nodes are cached to improve performance
+
+## Usage
+
+Here is how we evaluate arithmatic expressions.
+```java
+Object result = ScriptEngine.getInstance().eval('1 + 2 * (3 - 1)');
+```
+
+Or we can evaluate expressions consuming variables like this:
+```java
+Map<String, Object> context = new Map<String, Object>{
+    'a' => 1,
+    'b' => 2
+};
+Object result = ScriptEngine.getInstance().eval('a + b', context);
+```
+
+Or we can parse the nodes first, and the evaluate them:
+```java
+Jsep.Node node = new Jsep('1 + 2').parse();
+Object result = ScriptEngine.getInstance().eval(node);
+```
+
+## Supported Operations
+
+Supported Unary Operator
+
+| Name | Description |
+| ---- | ----------- |
+| - | Negate number |
+| ! | Negate boolean |
+| ++ | Only prefix supported |
+| -- | Only prefix supported |
+
+Supported Logical Operator
+
+| Name | Description |
+| ---- | ----------- |
+| && | Logical and |
+| \|\| | Logical or |
+
+Supported Binary Operator
+
+| Name | Description |
+| ---- | ----------- |
+| == | Apex == |
+| != | Apex != |
+| === | Apex == |
+| !== | Apex != |
+| < | Apex < |
+| > | Apex > |
+| <= | Apex <= |
+| >= | Apex >= |
+| + | Apex + |
+| - | Apex - |
+| * | Apex * |
+| / | Apex / |
+| % | Apex Math.mod |
+
+Supported Structure
+
+| Name | Description |
+| ---- | ----------- |
+| Conditional Expression(? :) | Yes |
+| Array Literal | Yes |
+| Object Literal | Yes |
+
+For example,
+```java
+ScriptEngine.getInstance.eval('["a", "b"]'); // Array literal
+
+ScriptEngine.getInstance.eval('{ "name": "test", age: 18 }'); // Object literal
+```
+
+## Todos
+
+### Assignment Expression Evaluation
+Assignment expressions can be parsed, but cannot be evaluated yet.
+
+### Function Invocation
+Functions(methods) can be parsed now, but invocation is not implemented yet.