Add how-to change provider namespace (#206)

close #122

docs/howtos/ARM/change-provider-namespace.md

@@ -0,0 +1,24 @@
+# Change the provider namespace
+
+If you want to have a different provider namespace than the TypeSpec namespec you'll have to change the following
+
+## 1. Change the name in the spec
+
+```tsp
+@armProviderNamespace("MyDifferent.ProviderName")
+namespace Microsoft.Contoso {
+
+}
+```
+
+## 2. Change the output configuration
+
+If you want to match the folder structure of the azure-specs repo you might have to replace the `{service-name}` interpolation with the name you changed.
+
+```diff
+options:
+  '@azure-tools/typespec-autorest':
+     # ... other options
+-    output-file: "{azure-resource-provider-folder}/{service-name}/{version-status}/{version}/xxx.json"
++    output-file: "{azure-resource-provider-folder}/MyDifferent.ProviderName/{version-status}/{version}/xxx.json"
+```