roles/nginx: Add comment for gzip_vary

From: https://github.com/h5bp/server-configs-nginx/blob/master/nginx.conf

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/nginx/files/nginx.conf

@@ -29,6 +29,11 @@ http {
     keepalive_timeout  65s;
 
     gzip on;
+
+    # Tell proxies to cache both the gzipped and regular version of a resource
+    # whenever the client's Accept-Encoding capabilities header varies;
+    # Avoids the issue where a non-gzip capable client (which is extremely rare
+    # today) would display gibberish if their proxy gave them the gzipped version.
     gzip_vary on;
     gzip_comp_level 6;
     gzip_min_length 860;