Add subgroups documentation

groups.md

@@ -32,17 +32,152 @@ would produce (simplified) HTML output something like this:
 ```html
 <fieldset>
     <legend>User Details</legend>
-    
+
     <label>Name</label>
     <input type="text" />
-    
+
     <label>Age</label>
     <input type="number" />
 </fieldset>
 ```
 
+## Tags
+
+### Default
+
 You can change the default `fieldset` tag by passing the `tag` property to the `<vue-form-generator />` component in your markup.
 
 ```html
 <vue-form-generator tag="div" :model="model" :schema="schema" :options="options"></vue-form-generator>
 ```
+
+### Per group
+
+You can also change the `fieldset` tag per group by adding a tag to the group definition:
+
+```js
+{
+    groups: [
+        {
+            tag: "section",
+            ...
+        }
+    ]
+}
+```
+
+## Subgroups
+
+You can create a subgroup by adding a `groups` definition inside of a top-level `groups` definition. This is useful for grouping sections of large forms.
+
+```js
+{
+    groups: [
+        {
+            legend: "User Details",
+            fields: [
+                {
+                    type: "input",
+                    inputType: "text",
+                    label: "Name",
+                    model: "name"
+                },
+                {
+                    type: "input",
+                    inputType: "number",
+                    id: "current_age",
+                    label: "Age",
+                    model: "age"
+                }
+            ],
+            groups: [
+                {
+                    legend: "Location",
+                    fields: [
+                        {
+                            type: "input",
+                            inputType: "text",
+                            label: "City"
+                        },
+                        {
+                            type: "input",
+                            inputType: "text",
+                            label: "State"
+                        }
+                    ]
+                }
+            ]
+        }
+    ]
+}
+```
+
+would produce (simplified) HTML output something like this:
+
+```html
+<fieldset>
+    <legend>User Details</legend>
+
+    <label>Name</label>
+    <input type="text">
+
+    <label>Age</label>
+    <input type="number">
+
+    <fieldset>
+        <legend>Location</legend>
+            <label>City</label>
+            <input type="text">
+
+            <label>State</label>
+            <input type="text">
+    </fieldset>
+</fieldset>
+```
+
+### Bootstrap Vue Example
+
+Using custom tags, subgroups, and the `legendAttr` option, forms can be grouped using custom Vue components. The `legendAttr` option will render the legend text into the attribute you specify. The below example uses bootstrap-vue to render subgroups in tabs:
+
+```js
+{
+    groups: [
+        {
+            tag: "b-tabs",
+            legend: "Additional Info",
+            groups: [
+                {
+                    tag: "b-tab",
+                    legend: "Location",
+                    legendAttr: "title",
+                    fields: [
+                        {
+                            type: "input",
+                            inputType: "text",
+                            label: "City"
+                        },
+                        {
+                            type: "input",
+                            inputType: "text",
+                            label: "State"
+                        }
+                    ]
+                },
+                {
+                    tag: "b-tab",
+                    legend: "Demographics",
+                    legendAttr: "title",
+                    fields: [
+                        {
+                            type: "input",
+                            inputType: "number",
+                            label: "Age",
+                            model: "age"
+                        }
+                    ]
+                }
+            ]
+        }
+    ]
+}
+```