Add instructions to readme

readme.md

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-Dynamictaxmode for Commerce
-------------------------
+# Dynamic Tax Mode for Commerce
 
-This is an extension for modmore's [Commerce](https://modmore.com/commerce/). It does ... something really awesome ... and is available under the MIT License.
+
+This is an extension for modmore's [Commerce](https://modmore.com/commerce/). 
+
+This module allows a product to switch between inclusive and exclusive tax modes during 
+checkout based on a session variable, and provides a custom product type called DynamicTaxModeProduct 
+that allows setting both normal pricing and business pricing. The pricing shown to the customer 
+depends on the tax mode currently active for that product.
+
+It is available under the MIT License.
 
 Requirements:
 
-- Commerce 1.3+
+- Commerce 1.5+
 - PHP 7.4+
-- ... other requirements ..
 
-To use, install the package from [the MODX|modmore package provider](.. link to extras page ..), and enable the module under Extras > Commerce > Configuration > Modules.
+To use, install the package from the [modmore package provider](https://modmore.com/about/package-provider/), and enable the module under Extras > Commerce > Configuration > Modules.
 
 ## Detailed usage
 
-..
+**Set the session key**
+
+The tax mode changes between `exclusive` and `inclusive` depending on a session value. The default
+key for the session variable is `commerce_dynamictaxmode`. If you would like to change it to something
+else, you can set it in the module configuration window. Commerce > Configuration > Modules.
+
+**Create products with business pricing**
+
+Create a new product with the type `DynamicTaxModeProduct`, or you could also edit a current product and 
+change to that type. Click on the pricing tab and you'll see fields for both normal pricing and business
+pricing. Enter the prices and hit save.
+
+**Set the session variable**
+
+If no session variable is set, Dynamic Tax Mode Products will fall back to using the global system setting
+`commerce.tax_calculation`. 
+
+Use your custom session key, or the default `commerce_dynamictaxmode`. The value should be set as either `inclusive` or `exclusive`. 
 
-## Credits
+A basic snippet on a product page might be one way of setting the session variable. e.g. 
+```
+<?php
+$_SESSION['commerce_dynamictaxmode'] = 'inclusive';
+return '';
+```
 
-..
+**Public methods**
 
-## Support
+`DynamicTaxModeProduct` includes two public methods to assist with getting and saving business pricing programmatically:
 
-Please open an issue with feature requests or bug reports. If you need help setting up or customising Dynamictaxmode, [paid support is available](.. link to a support page or email ..).
+- getBusinessPricing(comCurrency $currency): ?ItemPricingInterface
+- saveBusinessPricing(ItemPricingInterface $pricing): bool