Update Resource DSL doco to reflect lazy loading.

Signed-off-by: Ryan Davis <zenspider@chef.io>

docs/dsl_resource.md

@@ -98,3 +98,32 @@ end
 ```
 
 For a full example, see our [example resource](https://github.com/chef/inspec/blob/master/examples/profile/libraries/example_config.rb).
+
+## Lazy Loading
+
+Prior to InSpec v4.16, resources were pre-loaded for every invocation
+of `inspec`. This was a heavy and unnecessary burden on the system and
+exacerbated startup times (especially on Windows).
+
+As of InSpec v4.16, resources are lazily loaded into the `inspec`
+process upon use. This greatly speeds up the initial startup costs of
+the `inspec` process and only loads what you need to use (eg, `inspec
+--version` no longer runs for 10 seconds!).
+
+### Overriding Core Resources
+
+Lazy loading does change the way the resource registry is handled in
+ways that might break some assumptions. Specifically,
+`inspec.<resource>` isn't pre-populated with the core resources that
+InSpec ships with. If you make a local/custom resource of the same
+name, referring to the core resource via `inspec.<resource>` will not
+resolve to the core resource.
+
+As such, overriding core resources is not recommended best practice.
+
+If you really do need to do this, it is easiest to make a local
+resource with a new name and refer to the core resource directly.
+Otherwise, you need to ensure that the core resource you want is
+registered (via `require "inspec/resource/<name>"`) _before_ your
+profile is run to ensure it is eagerly loaded and in the global
+resource registry.