Add docs for mailchimp

Author: Mark-H

en/01_Commerce/v1/50_Modules/Mailchimp.md

@@ -0,0 +1,36 @@
+With the [Mailchimp Extension](https://modmore.com/commerce/extensions/mailchimp/) you can allow your customers to sign up to a Mailchimp mailing list (or audience) when placing an order. 
+
+## Setup
+
+First install the free extension from the modmore package provider. 
+
+Navigate to Extras > Commerce > Modules and find Mailchimp in the list. Open its configuration and enter your API Key; you can find this in Mailchimp by clicking on your name in the top right > Account > Extras > API Keys. 
+
+Save the api key on the module configuration, at which point you'll get some additional options on the module. 
+
+- **Mailchimp List** allows you to select the list to subcribe customers to.
+- **Address Type** lets you choose if you want to use the email address on the provided billing or shipping address. This will fallback to the other type if the preferred type is not set. 
+- **Enable Double Opt-in** will instruct Mailchimp to send a double opt-in email to the user to confirm their subscription. 
+
+Also very important: **add the opt-in markup to your template**. 
+
+## Add the required markup
+
+The module supports the cart, address, and payment method steps/templates, so you can choose which one you want to place the opt-in on:
+
+- `frontend/checkout/cart.twig`
+- `frontend/checkout/address.twig`
+- `frontend/checkout/payment-method.twig`
+
+(If you haven't used the templating in Commerce before, see [Front-end Theming for how to do that.](../Front-end_Theming))
+
+The module expects a POST value with the name `mailchimp_opt_in` and a value of `on`. You could for example use this markup, which will only be shown if the module is properly configured and the customer is not already subscribed:
+
+``` html
+{% if mailchimp_enabled and not mailchimp_subscribed %}
+    <label class="c-subscribe-newsletter">
+        <input type="checkbox" name="mailchimp_opt_in" value="on">
+        {{ lex('commerce_mailchimp.subscribe_to_newsletter') }}
+    </label>
+{% endif %}
+```

en/01_Commerce/v1/50_Modules/index.md

@@ -1,16 +1,43 @@
-Commerce uses Modules to provide most functionality and features. 
+---
+title: 'Modules & Extensions'
+---
 
-Many modules are shipped with the core Commerce package, requiring no further install, but third party modules are also available offering different payment providers, shipping partners, communication-related integrations or other features. 
+Commerce uses Modules to provide most functionality and features. 
 
-In this section of the documentation you'll find information about all modules that are available from modmore. Third party modules may be missing from the list, while we encourage third party developers to submit documentation to this site we can not force them to do so, and in some cases we may not accept these contributions until we have had a chance to review the module itself. 
+Many modules are shipped with the core Commerce package, requiring no further install.
 
-Choose one of the following categories to see the matching modules:
+Other modules are provided as [standalone extensions](https://modmore.com/commerce/extensions/), installable through the MODX Extras installer. There are extensions for payment gateways, shipping partners, communication-related integrations and more. 
 
-- Payments
-- Shipping
-- Communication
-- Taxes
-- Other
+[**All available extensions, including third party extensions, can be found here.**](https://modmore.com/commerce/extensions/)
 
-**Note**: not all the modules are actually be exposed as modules in the Commerce admin area. For example payment providers and shipping integrations that are provided with the Commerce package are not shown as modules, instead they are just available when creating a payment or shipping method. 
+In alphabetical order:
 
+- [Accept Terms](Cart/AcceptTerms)
+- [AutoFill GeoIP](Cart/AutoFillGeoIP)
+- [Basic Address Validation](Address_Validation/Basic)
+- [Basic Custom Fields](Custom_Fields_(Basic))
+- [CloudPrint](CloudPrint)
+- [Combine Products](Cart/CombineProducts)
+- [Country Validation](Address_Validation/Country)
+- [Coupons](Discounts/Coupons)
+- [Customers](Admin/Customers)
+- [Default Address](Cart/DefaultAddress)
+- [Dymo Address Label](Shipping/DymoAddressLabel)
+- [Enforce Stock](Cart/EnforceStock)
+- [EU VAT](Taxes/EUVat)
+- [EU VAT Validator](Address_Validation/EUVat_Validator)
+- [Gateways Pack 1](Payments/GatewayPack1)
+- [Hide Products](Admin/HideProducts)
+- [ItemData](Cart/ItemData)
+- [Mailchimp](Mailchimp)
+- [MessageBird](Communication/MessageBird)
+- [Minimum Order Amount](Cart/MinOrderAmount)
+- [mPDF Writer](mPDFWriter)
+- [Packing Slip](Shipping/PackingSlip)
+- [PDFCrowd Writer](PDFCrowdWriter)
+- [Table Rates](Shipping/TableRates)
+- [TaxJar](Taxes/TaxJar)
+- [Update Stock](Cart/UpdateStock)
+- [UserProfileAddress](Cart/UserProfileAddress)
+- [User Group Shipment](Shipping/UserGroupShipment)
+- [User & User Group Discounts](Discounts/UserDIscounts)