Add blurb about having the sheriff try to roll clang

Change-Id: I9d4971c6756e915db4beccbff330295438dc70b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410729Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/master@{#808177}

docs/clang_sheriffing.md

@@ -24,6 +24,9 @@ Ideally these tests are covered by upstream LLVM bots and breakages are
 quickly noticed and fixed by the original author of a breaking commit,
 but that is sadly not always the case.
 
+We try to update Clang (roughly) weekly. Time permitting, try to [update the
+compiler](updating_clang.md) when bots are green enough.
+
 [TOC]
 
 ## Disk out of space