Add a link to a Zulip thread.

Author: nnethercote

triage/2020-07-21.md

@@ -7,7 +7,8 @@ regressions, several of them ranging from large to huge, many in rollups. Some
 additional regressions may have occurred in rollups that were masked by other
 regressions/improvements. 3 improvements, one of which was a reversion of a
 regression. Unpicking all this is going to take days and I'm not confident
-we'll actually identify every regression.
+we'll actually identify every regression. A follow-up thread on Zulip is
+[here](https://rust-lang.zulipchat.com/#narrow/stream/247081-t-compiler.2Fperformance/topic/Follow-up.20to.20the.20bad.20perf.20triage.20of.202020-07-21)
 
 In better news, rustdoc performance is now being benchmarked, thanks to the
 efforts of [Joshua Nelson](https://github.com/rust-lang/rustc-perf/pull/675).