update cmake instructions

Author: melode11

README.md

@@ -19,9 +19,17 @@ By virtue of being written in C++, this client works in several different platfo
 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> ./`
-4. Run `make`(if makefile generated) or open generated project (if project file generated) to build.
+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.
 
+*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>
+./
+```
+
 ### Without 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.