Docs plugin readme (#20048)

* Update plugin README template to account for themes and be more explicit about package managers

* Update plugin template to account for themes and more specific installation instruction guidelines

* Update docs/contributing/docs-templates.md

Co-Authored-By: Aisha Blake <aisha.g.blake@gmail.com>

* Update docs/contributing/docs-templates.md

Co-Authored-By: Aisha Blake <aisha.g.blake@gmail.com>

docs/contributing/docs-templates.md

@@ -399,6 +399,8 @@ them at the bottom in an "Other Resources" section. You can also mention here an
 
 Include a summary of what this plugin accomplishes. Is there a demo site that shows how this plugin operates? If so, include a link to the deployed demo site and/or its source code here.
 
+Themes are considered plugins in the Gatsby ecosystem and should follow this README as well. Note that themes include multiple READMEs. One inside the theme directory with configuration instructions, one inside the example project directory, and one in the root of the repository which will follow this guide.
+
 ### Dependencies (optional)
 
 Are there any plugins that must be installed in order to make this plugin work? If so, please include a list of those plugins and links to their pages here.
@@ -411,6 +413,10 @@ If there are other tutorials, docs, and learning resources that are necessary or
 
 Please include installation instructions here.
 
+Gatsby documentation uses `npm` for installation. This is the recommended approach for plugins as well.
+
+If the plugin is a theme that needs to use `yarn`, please point to [the documentation for switching package managers](/docs/gatsby-cli/#how-to-change-your-default-package-manager-for-your-next-project) in addition to the `yarn`-based instructions.
+
 ## Available options (if any)
 
 ## When do I use this plugin?
@@ -431,6 +437,8 @@ This section could also include before-and-after examples of data when the plugi
 
 If this is a source plugin README, source plugins ought to allow people to query for data within their Gatsby site. Please include code examples to show how to query for data using your source plugin.
 
+If this is a theme that requires data in a specific format in order to match an existing query, include those examples here.
+
 ## How to run tests
 
 ## How to develop locally