Python currency support (#437)

* Adds page for python currency support docs

* Currency docs

* Add info on manual currency update

docs/api/python/currency.md

@@ -0,0 +1,56 @@
+---
+title: Python Currency Support
+---
+
+## Currency Support
+
+InvenTree provides native support for multiple currencies, which can mean that data require conversion between these currencies, at defined exchange rates.
+
+The InvenTree server maintains a set of exchange rates, which are updated periodically. These exchange rates are available via the [InvenTree API](../api.md), and can be used by the Python bindings.
+
+### CurrencyManager Class
+
+The Python bindings provide the `CurrencyManager` class, which takes care of retrieving currency exchange data from the server. This class can be instantiated as shown below:
+
+```python
+from inventree.currency import CurrencyManager
+
+# The manager class must be passed a valid InvenTreeAPI instance
+manager = CurrencyManager(api)
+
+# Access the 'base currency' data
+base_currency = manager.getBaseCurrency()
+
+# Access the 'exchange rate' data
+rates = manager.getExchangeRates()
+```
+
+### Currency Conversion
+
+Currency conversion is performed by passing the value of currency, as well as the *source* and *target* currency codes to the currency manager.
+
+!!! warning "Missing Currency Data"
+    The currency conversion only works if the manager class has valid information on both the *source* and *target* currency exchange rates!
+
+```python
+from inventree.currency import CurrencyManager
+
+manager = CurrencyManager(api)
+
+# Convert from AUD to CAD
+cad = manager.convertCurrency(12.54, 'AUD', 'CAD')
+
+# Convert from NZD to USD
+usd = manager.convertCurrency(99.99, 'NZD', 'USD')
+```
+
+### Exchange Rate Update
+
+To request a manual update of currency data (from the server), run the following command:
+
+```python
+from inventree.currency import CurrencyManager
+
+manager = CurrencyManager(api)
+manager.refreshExchangeRates()
+```