Document max_workers flag

README.md

@@ -51,6 +51,9 @@ We start by creating a config file to tell OctoDNS about our providers and the z
 
 ```yaml
 ---
+manager:
+  max_workers: 2
+
 providers:
   config:
     class: octodns.provider.yaml.YamlProvider
@@ -80,6 +83,8 @@ zones:
 
 Further information can be found in the `docstring` of each source and provider class.
 
+The `max_workers` key in the `manager` section of the config enables threading to parallelize the planning portion of the sync.
+
 Now that we have something to tell OctoDNS about our providers & zones we need to tell it about or records. We'll keep it simple for now and just create a single `A` record at the top-level of the domain.
 
 `config/example.com.yaml`