Clarifying what 0s means for default expiry time

MattLTW · MattLTW · commit 82e167397ef5 · 2016-02-10T11:47:04.000+01:00
diff --git a/README.markdown b/README.markdown
@@ -843,6 +843,8 @@ The `<time>` argument values are in seconds by default. But it's wise to always
 
 This time must be less than 597 hours.
 
+Setting an expire time of 0s will cache the content forever.
+
 This directive was first introduced in the `v0.12rc7` release.
 
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