Add instructions for admins to achieve Pinafore compatibility (#406)

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docs/Admin-Guide.md

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+Admin Guide
+====
+
+This guide is for instance admins who may be having trouble with Pinafore compatibility.
+
+By default, [Mastodon allows cross-origin access to the `/api` endpoint](https://github.com/tootsuite/mastodon/blob/50529cbceb84e611bca497624a7a4c38113e5135/config/initializers/cors.rb#L15-L20). Thus Pinafore should "just work" for most Mastodon servers.
+
+If the nginx/Apache settings have been changed, though, then Pinafore might not be able to connect to an instance. To check if the instance is supported, run this command:
+
+```bash
+curl -sLv example.com/api/v1/instance -o /dev/null
+```
+
+If you see this in the output:
+
+```
+Access-Control-Allow-Origin: *
+```
+
+Then Pinafore should work as expected!
+
+Otherwise, if the instance admin would like to whitelist only certain websites (including Pinafore) to work with CORS, then they will need to make sure that the server echoes: 
+
+```
+Access-Control-Allow-Origin: https://pinafore.social
+```
+
+when Pinafore tries to access it. Note that this is a bit complicated to configure (compared to the simpler `*` approach), but [there are instructions on StackOverflow](https://stackoverflow.com/q/1653308) for nginx, Apache, and other servers.