add English doc for ngx_http_reqstat_module

docs/modules/ngx_http_reqstat_module.md

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+Name
+====
+
+* ngx_http_reqstat_module
+
+Description
+===========
+
+This module will help monitor running status of Tengine.
+
+* It can provide running status information of Tengine.
+
+* The information is divided into different zones, and each zone is independent.
+
+* The status information is about connections, requests, response status codes, input and output flows,
+  rt, and upstreams.
+
+* It shows all the results by default, and can be set to show part of them by specifying zones.
+
+Compilation
+===========
+
+The module is compiled into Tengine by default. It can be disabled with '--without-http_reqstat_module'
+configuration parameter, or it can be compiled as a '.so' with '--with-http_reqstat_module=shared'.
+
+
+Example
+===========
+
+    http {
+        req_status_zone server "$host,$server_addr:$server_port" 10M;
+
+        server {
+            location /us {
+                req_status_show;
+            }
+
+            req_status server;
+        }
+    }
+
+* when you call '/us', you will get the results like this:
+
+            www.taobao.com,127.0.0.1:80,162,6242,1,1,1,0,0,0,0,10,1,10,1
+
+    * Each line shows the status infomation of a "$host,$server_addr:$server_port".
+
+    * Line format:
+
+            kv,bytes_in_total,bytes_out_total,conn_total,req_total,2xx,3xx,4xx,5xx,other,rt_total,upstream_req,upstream_rt,upstream_tries
+
+        * **kv**                value of the variable defined by the directive 'req_status_zone'
+        * **bytes_in_total**    total number of bytes received from client
+        * **bytes_out_total**   total number of bytes sent to client
+        * **conn_total**        total number of accepted connections
+        * **req_total**         total number of processed requests
+        * **2xx**               total number of 2xx requests
+        * **3xx**               total number of 3xx requests
+        * **4xx**               total number of 4xx requests
+        * **5xx**               total number of 5xx requests
+        * **other**             total number of other requests
+        * **rt_total**          accumulation or rt
+        * **upstream_req**      total number of requests calling for upstream
+        * **upstream_rt**       accumulation or upstream rt
+        * **upstream_tries**    total number of times calling for upstream
+
+* tsar can parse the result and monitor, see also https://github.com/alibaba/tsar
+
+Directives
+==========
+
+req_status_zone
+-------------------------
+
+**Syntax**: *req_status_zone zone_name value size*
+
+**Default**: *none*
+
+**Context**: *main*
+
+create shared memory for this module. 'zone_name' is the name of memory block.
+'value' defines the key, in which variables can be used.
+'size' defines the size of shared memory.
+
+Example:
+
+    req_status_zone server "$host,$server_addr:$server_port" 10M;
+
+    the memory is 10MB, the key is "$host,$server_addr:$server_port", and the name is "server".
+
+* Notice, if you want to use tsar to monitor, you should not use comma in the key.
+
+
+req_status
+-------------------------
+
+**Syntax**: *req_status zone_name1 [zone_name2 [zone_name3]]*
+
+**Default**: *none*
+
+**Context**: *main、srv、loc*
+
+Enable monitoring. You can specify multiple zones to monitor.
+
+req_status_show
+-------------------------
+
+**Syntax**: *req_status_show [zone_name1 [zone_name2 [...]]]*
+
+**Default**: *all the targets defined by 'req_status_zone'*
+
+**Context**: *loc*
+
+Display the status information. You can specify zones to display.