doc: added a note about the empty values of $echo_client_request_headers in HTTP/2 requests.

agentzh · agentzh · commit 477c872b4749 · 2016-08-26T12:10:08.000-07:00
diff --git a/README.markdown b/README.markdown
@@ -1493,6 +1493,8 @@ Accessing `/echoback` yields
 
 Behind the scene, it recovers `r->main->header_in` (or the large header buffers, if any) on the C level and does not construct the headers itself by traversing parsed results in the request object.
 
+This varible is always evaluated to an empty value in HTTP/2 requests for now due to the current implementation.
+
 This variable was first introduced in [version 0.15](#v015).
 
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