Add doc about how to consume CI builds

doc/consume_ci.md

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+# Testing fixes prior to release
+
+To help us test changes prior to their release to nuget.org, you can of course build from source.
+An easier approach may be to consume the built nuget packages from our CI/PR feed.
+
+Add this feed to your nuget.config file:
+
+```xml
+<add key="MessagePack-CI" value="https://pkgs.dev.azure.com/ils0086/MessagePack-CSharp/_packaging/MessagePack-CI/nuget/v3/index.json" />
+```
+
+Then you can add or update your package reference to some version recently built in our CI or PR build.
+PR builds always include a `-gCOMMITID` version suffix.
+CI builds lack this, but may include a standard pre-release identifier such as `-alpha`.
+
+If the change you seek is already merged, look for the latest version without the `-gCOMMITID` suffix.
+If the change you seek is in an open PR, navigate to the PR build to find the version of the built package (it will be the build number).