Update CONTRIBUTING.md (#1200)

Add `addUsesTo` section under enums

CONTRIBUTING.md

@@ -84,6 +84,7 @@ Enums are defined in [enums.json](generation/WinSDK/enums.json). This file provi
 * `type` - Optional property indicating the type for the enum
   * Note: If omitted, the default value is `uint`
 * `name` -  The name of the enum
+* `addUsesTo` - The name of an existing enum scraped from the headers, specify this value to add additional uses to the enum.
 * `flags` - Whether this is a flags enum
 * `autoPopulate` - For automatically defined members, rules for how to populate them
   * `header` - The header to scan
@@ -199,4 +200,4 @@ When validating changes, it's important to evaluate the diffs to ensure all chan
 
 Additionally, it is useful to load the winmd in [ILSpy](https://github.com/icsharpcode/ILSpy) and navigate through the APIs as another means to identify additional changes that may be required to achieve the desired end result. You may notice that two related APIs are in different namespaces or that a type that an API depends on was not moved as you would have expected. If that happens, search the repo for the API or its header file to identify where it may be being mapped to another namespace.
 
-Once all the changes are validated, update the list of known changes since the last release by following the steps reported in the build output. When a new release is made, the list of changes in [ChangesSinceLastRelease.txt](scripts/ChangesSinceLastRelease.txt) will get reset and will start accumulating again until the next release.
+Once all the changes are validated, update the list of known changes since the last release by following the steps reported in the build output. When a new release is made, the list of changes in [ChangesSinceLastRelease.txt](scripts/ChangesSinceLastRelease.txt) will get reset and will start accumulating again until the next release.