docs: add formula calculation documentation

formula-readme.md

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+## Formula Calculation
+
+### Instalation
+```
+yarn add mathjs algebra.js
+```
+
+
+### Example
+```ts
+import * as math from 'mathjs'
+import { Equation, parse } from 'algebra.js'
+
+const formula = 'dpp - (dpp*ppn) - (dpp*retribusi) - (dpp*service) - (dpp*ppn3)'
+const total = '300000'
+
+const variable = {
+  ppn: 11,
+  retribusi: 5000,
+  service: 5,
+  ppn3: 5000
+}
+
+try {
+  
+  const x1 = math.simplify(formula, variable).toString()
+  console.log('Formula ', x1)
+  const dppFormula = parse(x1)
+  const totalFormula = parse(total)
+  const equation = new Equation(totalFormula, dppFormula)
+
+  console.log(equation.toString())
+  const result = equation.solveFor('dpp').toString()
+  console.log(result)
+  
+  const value = math.evaluate(result)
+  console.log(value)
+
+} catch (e) {
+  console.log(e)
+}
+```