doc: added doc of limit count.

lua/apisix/plugins/limit-count.md

@@ -0,0 +1,23 @@
+# limit-count
+
+* `conn` is the maximum number of concurrent requests allowed. Requests exceeding this ratio (and below `conn` + `burst`)
+will get delayed to conform to this threshold.
+* `burst` is the number of excessive concurrent requests (or connections) allowed to be delayed.
+* `rejected_code` is the response code when the current request was rejected.
+* `key` is the user specified key to limit the concurrency level.`
+
+Here is an example of binding to route:
+
+```json
+ {
+     "uri": "/hello",
+     "plugin_config": {
+         "limit-count": {
+             "count": 100,
+             "time_window": 600,
+             "rejected_code": 503,
+             "key": "remote_addr"
+         }
+     }
+ }
+```