Process doc for develop branch

docs/develop-branch.md

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+`develop` Branch
+===
+
+Building the full Windows App SDK package involves multiple repos and build pipelines.
+Beyond the publicly-visible repos, we also have closed-source repos that produce public packages which
+are then consumed by this repo ([WindowsAppSDK](https://github.com/microsoft/WindowsAppSDK)).
+
+Some of those packages can't yet be pushed to the public feed on a CI/nightly basis though.
+Until that's resolved, we could get this repo into a state of being buildable only internally,
+by restoring the latest package from an internal feed.
+
+To allow either configuration to be built (using the public dependent package or the internal one),
+we maintain two branches (similar to the GitFlow process):
+
+* `main` branch consumes the latest public package
+* `develop` branch consumes the latest internal package.
+
+The `develop` branch code is in the repo and still open, but can only be built internally.
+The `main` branch is always buildable at HEAD.
+External contributions to the repo are made to the `main` branch,
+and are merged internally into `develop` when completed.
+
+Whenever a new public version of the dependent packages are pushed to the public feed,
+the `develop` branch can be merged into `main`.
+For example as part of (pre)releases.
+
+Merging between `develop` and `main` is normal (not squashed),
+so that both branches can show the correct history, just with a few extra merge commits.
+Once the packages can be kept immediately updated we can drop the `develop` branch
+and not lose any history.
+
+The long-lasting difference between `main` and `develop` is the `nuget.config` file;
+in `main` it references the public feed and in `develop` it references the internal feed.
+Package references might be different too if `develop` branch is consuming a package that's
+not yet on the public feed.
+
+![Example of main and develop branching](images/develop-branch-example.jpg)