Document the process for making clang plugin behavior changes.

Change-Id: I7f35f8125cbf1bb1bd4896e47ae6f77208d0ec90
Reviewed-on: https://chromium-review.googlesource.com/986885Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546945}

docs/clang.md

@@ -49,6 +49,17 @@ To test the FindBadConstructs plugin, run:
      ./test.py ../../../../third_party/llvm-build/Release+Asserts/bin/clang \
                ../../../../third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so)
 
+Since the plugin is rolled with clang changes, behavior changes to the plugin
+should be guarded by flags to make it easy to roll clang. A general outline:
+1.  Implement new plugin behavior behind a flag.
+1.  Wait for a compiler roll to bring in the flag.
+1.  Start passing the new flag in `GN` and verify the new behavior.
+1.  Enable the new plugin behavior unconditionally and update the plugin to
+    ignore the flag.
+1.  Wait for another compiler roll.
+1.  Stop passing the flag from `GN`.
+1.  Remove the flag completely.
+
 ## Using the clang static analyzer
 
 See [clang_static_analyzer.md](clang_static_analyzer.md).