add README

README.md

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+# User Sync Tool Documentation
+
+This documentation is built with [Hugo](https://gohugo.io/). To build the site
+for local development:
+
+```
+$ hugo serve -D
+```
+
+This watches all files in the `content` directory and incrementally builds the
+site when changes are detected. When working with non-English languages, this
+can sometimes render pages improperly or crash the server. If that happens, kill
+the server if it's still running (`ctrl+c`), start it again and refresh the
+page.
+
+**NOTE:** The root documentation branch, `user-guide`, does not share commit
+history with `v2` or any other source code branches. We recommend cloning this
+branch to a separate directory. Switching between `user-guide`-derived branches
+and `v2`-derived branches in the same workspace may have undesirable side effects.
+
+To clone `user-guide` to a separate directory:
+
+```
+$ git clone -b user-guide https://github.com/adobe-apiplatform/user-sync.py.git user-sync.py-docs
+```