docs(readme): update docs

README.md

@@ -8,9 +8,26 @@ npm install --save eval-num-expr
 
 ## Usage
 
-```
-import evalExpr from "eval-num-expr"
+```js
+import evalExpr from "eval-num-expr";
 
-const value = evalExpr('100 + 3 * 55 + 73 - 73 * 18%')
+const value = evalExpr("100 + 3 * 55 + 73 - 73 * 18%");
 // value = 324.86
 ```
+
+You can use basic operations (e.g. +, -, \*, /, %, ^ etc.) in you expressions.
+
+### Examples
+
+- 175 + 175 \* 18% + 40 = 246.5
+- 10 + (100 - 100 \* 18%) \* 5 + 40 / 20 = 422
+
+## Api
+
+### evalExpr
+
+This is the default and only from this package.
+
+```js
+evalExpr(expressions: string, digitsAfterDecimal?: number = 2)
+```