doc(@embark/ens): document $accounts for ENS configuration

Adds docs for $accounts in subdomain registers and improve the docs

site/source/docs/naming_configuration.md

@@ -8,9 +8,11 @@ We can configure different naming systems in Embark. In this guide we'll explore
 
 Embark checks our configuration in `config/namesystem.js` by default. A naming system configuration isn't crucial to run Embark, so this only needs to be enabled when planing to use a naming system.
 
-When using ENS as our provider, we can set the `register` section to pre-register sub-domains. This feature is  only available in the development environment:
+When using ENS as our provider, we can set the `register` section to pre-register sub-domains.
 
-```
+This feature is  only available in the development environment:
+
+```javascript
 module.exports = {
   default: {
     enabled: true,
@@ -28,3 +30,34 @@ module.exports = {
   }
 };
 ```
+
+### Parameters
+
+- `rootDomain`: The ENS domain. It gets registered using your default account
+- `subdomains`: Object were the key is the subdomain name and the value is the address to set to it
+
+## Special configurations
+
+### $Contracts
+
+For subdomains, you can set the address as one of your contracts address using it's name prefixed by a dollar sign (`$`).
+
+```
+subdomains: {
+  'contract': '$MyContract'
+}
+```
+
+Now, assuming your `rootDomain` is `embark.eth`, `contract.embark.eth` will resolve to the deployed address of MyContract.
+
+### $accounts
+
+Similarly to `$Contract`, using `$accounts` let's you set the subdomain address to one of your accounts address.
+
+```
+subdomains: {
+  'account': '$accounts[0]'
+}
+```
+
+Now, assuming your `rootDomain` is `embark.eth`, `account.embark.eth` will resolve to address of your first (index 0) account (aka `defaultAccount`).