doc: add doc for watchcancel (#82)

Author: Yiyiyimu

Makefile

@@ -10,7 +10,7 @@ install:
 ### dev:          Create a development ENV
 .PHONY: dev
 dev:
-	luarocks install rockspec/lua-resty-etcd-master-0.1-0.rockspec --only-deps
+	luarocks install rockspec/lua-resty-etcd-master-0.1-0.rockspec --only-deps --local
 
 ### help:         Show Makefile rules
 .PHONY: help

README.md

@@ -28,7 +28,7 @@ luarocks install lua-resty-etcd
 > install by source
 
 ```shell
-$ luarocks install lua-resty-http lua-typeof
+$ make dev
 $ git clone https://github.com/iresty/lua-resty-etcd.git
 $ cd lua-resty-etcd
 $ sudo make install

api_v3.md

@@ -9,7 +9,7 @@ API V3
     * [setx](#setx)
     * [delete](#delete)
     * [watch](#watch)
-    * [watchdir](#watchdir)
+    * [watchcancel](#watchcancel)
     * [readdir](#readdir)
     * [watchdir](#watchdir)
     * [rmdir](#rmdir)
@@ -154,6 +154,7 @@ local res, err = cli:delete('/path/to/key')
     * `prev_kv`: (bool) If prev_kv is set, created watcher gets the previous KV before the event happens. If the previous KV is already compacted, nothing will be returned.
     * `watch_id`: (int) If watch_id is provided and non-zero, it will be assigned to this watcher. Since creating a watcher in etcd is not a synchronous operation, this can be used ensure that ordering is correct when creating multiple watchers on the same stream. Creating a watcher with an ID already in use on the stream will cause an error to be returned. 
     * `fragment`: (bool) fragment enables splitting large revisions into multiple watch responses.  
+    * `need_cancel`: (bool) if watch need to be cancel, watch would return http_cli for further cancelation. See [watchcancel](#watchcancel) for detail.
 
 Watch the update of key.
 
@@ -163,6 +164,21 @@ local res, err = cli:watch('/path/to/key')
 
 [Back to TOP](#api-v3)
 
+### watchcancel
+
+`syntax: res, err = cli:watchcancel(http_cli:table)`
+
+* `http_cli`: the http client needs to revoke.
+
+Cancel the watch before it got expired. Need to set `need_cancel` to get the http client for cancelation.
+
+```lua
+local res, err, http_cli = cli:watch('/path/to/key', {need_cancel = true})
+res = cli:watchcancel(http_cli)
+```
+
+[Back to TOP](#api-v3)
+
 ### readdir
 
 `syntax: res, err = cli:readdir(dir:string [, opts:table])`