Outdated information in Readme

[Menkalian]

The README states that "CMake with an equal or higher version than 2.8.5 is supported" but the project now sets cmake_minimum_required(VERSION 3.5) in CMakeLists.txt. This mismatch can cause users with CMake versions >= 2.8.5 and < 3.5 to follow the README instructions and fail when running cmake.

Relevant file:
README.md

90   #### CMake
91   
92   With CMake, cJSON supports a full blown build system. This way you get the most features. CMake with an equal or higher version than 2.8.5 is supported. With CMake it is recommended to do an out of tree build, meaning the compiled files are put in a directory separate from the source files. So in order to build cJSON with CMake on a Unix platform, make a `build` directory and run CMake inside it.
93   
94   ```
95   mkdir build
96   cd build
97   cmake ..
98   ```
99   
     ...
124  If you are packaging cJSON for a distribution of Linux, you would probably take these steps for example:
125  ```
126  mkdir build
127  cd build
128  cmake .. -DENABLE_CJSON_UTILS=On -DENABLE_CJSON_TEST=Off -DCMAKE_INSTALL_PREFIX=/usr
129  make
130  make DESTDIR=$pkgdir install
131  ```