docs: update README.md

README.md

@@ -4,6 +4,44 @@ Thai personal income tax utility functions.
 ### Background
 Thai tax calculation has proven to be difficult. Instead of extensive words, it would be simpler as programming code.
 
+### Usage
+```ts
+
+const thaiTax = new ThaiTax(2565)
+  .setIncome({
+    salary: 12 * 1_000_000,
+    bonus: 1_000_000,
+    other: 1_000_000,
+  })
+  .setGeneralInfo({
+    sso: 0,
+    houseInterest: 0,
+    socialEnterprise: 0,
+    shopDeeMeeKuen: 0,
+  })
+  .setFamily({
+    status: FamilyStatus.SINGLE,
+    noOfChildren: 1
+  })
+  .setInsurance({
+    life: 100_000,
+    health: 25_000,
+  })
+  .setFund({
+    ssf: 200_000,
+    rmf: 300_000,
+  })
+  .setDonation({
+    special: 1000,
+  })
+  .summarize();
+
+/**
+ * { netIncome: 13208000, tax: 4137800 }
+ **/
+
+```
+
 ### TO-DO
 - [ ] recheck logic
 - [ ] add Thai documentations (reference would be good)