Merge pull request #280 from cdjingit/master

添加wrr的文档说明

docs/cn/client.md

@@ -152,6 +152,10 @@ int main() {
 
 即round robin，总是选择列表中的下一台服务器，结尾的下一台是开头，无需其他设置。比如有3台机器a,b,c，那么brpc会依次向a, b, c, a, b, c, ...发送请求。注意这个算法的前提是服务器的配置，网络条件，负载都是类似的。
 
+### wrr
+
+即weighted round robin, 根据服务器列表配置的权重值来选择服务器。服务器被选到的机会正比于其权重值，并且该算法能保证同一服务器被选到的结果较均衡的散开。
+
 ### random
 
 随机从列表中选择一台服务器，无需其他设置。和round robin类似，这个算法的前提也是服务器都是类似的。

docs/en/client.md

@@ -152,6 +152,10 @@ The ideal algorithm is to make every request being processed in-time, and crash
 
 which is round robin. Always choose next server inside the list, next of the last server is the first one. No other settings. For example there're 3 servers: a,b,c, brpc will send requests to a, b, c, a, b, c, … and so on. Note that presumption of using this algorithm is the machine specs, network latencies, server loads are similar.
 
+### wrr
+
+which is weighted round robin. Choose the next server according to the configured weight. The chances a server is selected is consistent with its weight, and the algorithm can make each server selection scattered.
+
 ### random
 
 Randomly choose one server from the list, no other settings. Similarly with round robin, the algorithm assumes that servers to access are similar.