Add docs for custom templates option

app/pages/docs/cli-generate.mdx

@@ -105,6 +105,49 @@ app/tasks/mutations/updateTask.ts
 Note that this will not generate the relationships between the models,
 only the queries, mutations and pages.
 
+##### `--templateDir`
+
+Shorthand: `-t`
+
+Allows you to specify a custom template directory. If you want to use
+custom templates insetad of the default ones (e.g. with different styles),
+you can pass a path to the local template directory. The template
+directory should have the following structure:
+
+```bash
+.
+├── form
+│   └── __ModelName__Form.tsx
+├── mutation
+│   └── __input__.ts
+├── mutations
+│   ├── create__ModelName__.ts
+│   ├── delete__ModelName__.ts
+│   └── update__ModelName__.ts
+├── page
+│   ├── __modelIdParam__
+│   │   └── edit.tsx
+│   ├── __modelIdParam__.tsx
+│   ├── index.tsx
+│   └── new.tsx
+├── queries
+│   ├── get__ModelName__.ts
+│   └── get__ModelNames__.ts
+└── query
+    └── __name__.ts
+```
+
+If some folders are omitted in your custom template directory,
+`blitz generate` will fallback to the default templates. You can check
+them out
+[here](https://github.com/blitz-js/blitz/tree/canary/packages/generator/templates).
+
+**Usage:**
+
+```bash
+blitz generate all projects -t ./my-custom-templates
+```
+
 ##### `--dry-run`
 
 Shorthand: `-d`