Add version pin warning to common/

Could be added to README.md, but who reads those...

BUG=b:229016539
TEST=None

Change-Id: I3cc0f3672999926018fae572d8966dd971dcf80f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3586142
Reviewed-by: Allen Webb <allenwebb@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

common/chromeos_warning.md

@@ -0,0 +1,25 @@
+# Warning
+
+These crates are shared with ChromeOS and used by other ChromeOS Rust projects. However since crosvm
+is refactoring this part of the code heavily, ChromeOS has pinned the version of some of these
+crates:
+
+- cros_async
+- data_model
+- io_uring
+- sync
+- sys_util
+- sys_util_core
+
+Modifications made here will not be available for other ChromeOS projects.
+
+If you are looking for a place to add new ChromeOS-specific utilities, please consider
+[libchromeos-rs](https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/libchromeos-rs).
+
+If modifications to these crates cannot be avoided, consider manually updating the revision in the
+corresponding ebuild file, which may come with a significant need for refactoring.
+
+Alternatively, for small changes consider adding a patch file into the corresponding ebuild file
+instead.
+
+See [b/229016539](http://b/229016539) for details.