Add comment about fixing merge conflicts

When working with downstream branches chained to upstream branches, it
helps to use `git rebase -i @{u}` twice to pull upstream changes into
the downstream branch. Expect a lot of merge conflicts when you do
this, as git is often not smart enough to figure out which change
happened more recently, and should take precedence.

Bug: None
Change-Id: Ic742439981d6d63e8d6bd1e10776fa920c88eb7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341387
Reviewed-by: Michael Giuffrida <[email protected]>
Commit-Queue: Toby Huang <[email protected]>
Cr-Commit-Position: refs/heads/master@{#796561}
diff --git a/docs/commit_checklist.md b/docs/commit_checklist.md
index f2fa823..3ca96e7 100644
--- a/docs/commit_checklist.md
+++ b/docs/commit_checklist.md
@@ -48,6 +48,9 @@
 *   Run `git rebase -i @{u}` again to rebase the downstream changes onto the
     upstream branch.
 
+Expect to fix numerous merge conflicts. Use `git rebase --continue` once you're
+done.
+
 ## 3. Make your changes
 
 Do your thing. There's no further advice here about how to write or fix code.