Add meson documentation

[MaxBrandtner]

The following changes to the README file add documentation, how to use cJSON in a meson project. This would resolve #760.

[Alanscut]

Hi @MaxBrandtner
Thanks for your contribution. :)

I'm not that familiar with Meson. Is it a framework used to compile and build cjson, like CMake? It looks you only added cjson as a dependency, instead of building cjson.

Besides this, I noticed that example and example.c are also added. Are they releated to cjson?

[MaxBrandtner]

Meson is a build system. It has a configuration file meson.build which is the file that I described above. cjson is the variable that holds the dependency libcjson. Because we usually use more than one dependency when programming, the dependenies are stored in a dependency var (which I named cjson) to make it less tedious to debug the execution parameter. Perhaps I should have used a more descriptive name like dependency_list. Furthermore, the source files are usually kept in a sourcefiles variable, which I didn't do, but should have done for the sake of consistency. example.c is the source file(it could carry any other name or be a collection of source files), while example is the executable(which can have any name you want it to have) which is stored in the build directory. The build directory is specified when building the program so if you want to have a build dir .build you would compile the program in the following way.

meson setup .build
ninja -C .build

# execute compiled code
./.build/example

Perhaps a more generalized way would be:
meson.build

project('program-example', 'c')

dependency_list = dependency('libcjson')
sourcefiles = files('example.c')

executable(
    'example',
    sourcefiles,
    dependencies:  dependency_list,
)

[Alanscut]

Thanks for your detailed information! :)
This PR looks good to me now.