Add BOOST_VER cmake option in README

Author: melode11

README.md

@@ -18,15 +18,16 @@ By virtue of being written in C++, this client works in several different platfo
 ### With CMake
 1. Install boost, see [Boost setup](#boost_setup) section.
 2. Use `git clone --recurse-submodules https://github.com/socketio/socket.io-client-cpp.git` to clone your local repo.
-3. Run `cmake -DBOOST_PATH:STRING=<your boost install folder> ./`
+3. Run `cmake -DBOOST_PATH:STRING=<your boost install folder> -DBOOST_VER:STRING=<your boost version> ./`
 4. Run `make install`(if makefile generated) or open generated project (if project file generated) to build.
-5. Include `sio_client.h` in your client code where you want to use it.
+5. Find headers and lib under `./build`, link with the static lib and  include `sio_client.h` in your client code where you want to use it.
 
 *If you're using boost without install,you can specify `boost include dir` and `boost lib dir` separately by:* 
 ```bash
 cmake
 -DBOOST_INCLUDE:STRING=<your boost include folder> 
 -DBOOST_LIB:STRING=<your boost lib folder>
+-DBOOST_VER:STRING=<your boost version>
 ./
 ```