Explicitly document googletest release tag format

Fixes #4235

PiperOrigin-RevId: 532446854
Change-Id: If7e1dade02a80d9a30813c136bba3f20c796f0fa
diff --git a/README.md b/README.md
index 14bb9e3..a3bd4bd 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,8 @@
 [Abseil Live at Head philosophy](https://abseil.io/about/philosophy#upgrade-support).
 We recommend
 [updating to the latest commit in the `main` branch as often as possible](https://github.com/abseil/abseil-cpp/blob/master/FAQ.md#what-is-live-at-head-and-how-do-i-do-it).
+We do publish occasional semantic versions, tagged with
+`v${major}.${minor}.${patch}` (e.g. `v.1.13.0`).
 
 #### Documentation Updates