Add warning to preview build documentation

A warning has been added to the preview build documentation about a
known problem with that workflow. The warning explains the workaround
for the problem.

docs/contributing.md

@@ -21,6 +21,14 @@
 
 When developing changes to packages within this repository that a different project depends upon, you may wish to load those changes into the project and test them locally or in CI before publishing proper releases of those packages. To solve that problem, this repository provides a mechanism to publish "preview" versions of packages to GitHub Package Registry. These versions can then be used in the project like any other version, provided the project is configured to use that registry.
 
+> **Warning**
+>
+> There is a known problem with the preview build workflow. It relies upon you having a local cache of any non-preview `@metamask/`-scoped packages.
+>
+> If you encounter problems installing non-preview `@metamask/`-scoped packages when using this workflow, you can workaround the problem by temporarily removing the `.npmrc` / `.yarnrc.yml` changes to install the missing packages. Once they are installed, restore the preview build credentials to use preview builds. The non-preview `@metamask/`-scoped packages should then be found in your local cache.
+>
+> See [issue #1075](https://github.com/MetaMask/core/issues/1075) for more details.
+
 ### As a MetaMask contributor
 
 If you're a MetaMask contributor, you can create these preview versions via draft pull requests: