Added extending docs (#414)

.vitepress/config.ts

@@ -85,6 +85,10 @@ function getGuideSidebar() {
       text: 'Migrations',
       link: '/guide/migrations'
     },
+    {
+      text: 'Extending',
+      link: '/guide/extending'
+    }
   ]
 }
 function getFaqSidebar() {

src/guide/extending.md

@@ -0,0 +1,44 @@
+# Extending
+
+To extend knex's builders, we have the following methods
+
+```js
+knex.SchemaBuilder.extend("functionName", function() {
+    console.log('Custom Schema Builder Function');
+    return this;
+});
+knex.TableBuilder.extend("functionName", function() {
+    console.log('Custom Table Builder Function');
+    return this;
+});
+knex.ViewBuilder.extend("functionName", function() {
+    console.log('Custom View Builder Function');
+    return this;
+});
+knex.ColumnBuilder.extend("functionName", function() {
+    console.log('Custom Column Builder Function');
+    return this;
+});
+```
+
+
+To add typescript support you can add the following (.d.ts):
+```ts
+import "knex";
+declare module "knex" {
+    namespace Knex {
+        interface SchemaBuilder {
+            functionName (): Knex.SchemaBuilder;
+        }
+        interface TableBuilder {
+            functionName (): Knex.TableBuilder;
+        }
+        interface ViewBuilder {
+            functionName (): Knex.ViewBuilder;
+        }
+        interface ColumnBuilder {
+            functionName (): Knex.ColumnBuilder;
+        }
+    }
+}
+```