Add a stub article that summarizes Solidus's built-in tax calculator

guides/calculators/tax-calculator.md

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+# Tax calculator
+
+Solidus comes with a tax calculator that is used to calculate both sales tax
+(United States-style taxes) and value-added tax (VAT):
+[`Spree::Calculator::DefaultTax`][default-tax-calculator]. Typically, this
+calculator should be the only tax calculator required by your store.
+
+Using this calculator, all tax rates are represented as a decimal. So, a tax
+rate of 5% should be represented as `0.05`.
+
+Taxes can apply to line items, shipments, or an entire order.
+
+The tax calculator uses its calculable – a `Spree::TaxRate` – to calculate tax
+totals.
+
+For more comprehensive documentation about taxes in Solidus, see the
+[Taxation][taxation] documentation.
+
+If your store's tax requirements are more complicated, you may want to create a
+[custom tax calculator][custom-tax-calculator] or use an extension like
+[`solidus_tax_cloud`][solidus-tax-cloud].
+
+[custom-tax-calculator]: ../taxation/custom-tax-calculators.md
+[default-tax-calculator]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/calculator/default_tax.rb
+[solidus-tax-cloud]: https://github.com/solidusio-contrib/solidus_tax_cloud
+[taxation]: ../taxation/overview.md
+