RADME: add Use Cases section

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Change-Id: Iab619321d18776e186356c0d8d4a99b5e8728333

README.md

diff --git a/README.md b/README.md
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--- a/README.md
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@@ -32,6 +32,25 @@
 outgoing, incoming and CCed changes at the
 [dashboard](https://linux-review.googlesource.com/dashboard/self).
 
+## Use Cases
+
+There are several potential use cases:
+
+* **self-review**: upload a change just to double check the handy side-by-side
+  diffs
+* **pre-review**: you may do a pre-review within a team before sending the
+  change upstream for the official review; or use it for some preliminary design
+  discussions; or just keep track of progress and versioning
+* **complement the current review process**: include a link to the change
+  into the upstream patch email as FYI (maybe after "---" line); this preserves
+  the current process, but gives side-by-side diffs, full context,
+  version diffs, base commit, change tracking, etc
+
+Feel free to use it if you find it useful.
+
+**Note**: when you upload a new change nobody is notified. You can add
+reviewers/CC manually and/or give the change link to other people.
+
 ## Tutorial
 
 To create a change for the