File Browser: Depends on GLEW

Platform: MacOS Big Sur
Xcode: Version 12.4

From the ``example`` folder I run:

```
make
```

I get this error:

```
file_browser.c:14:10: fatal error: 'GL/glew.h' file not found
#include
^~~~~~~~~~~
```

It appears that I need to have bunch of other things installed before I can use and build with Nuklear. I don't see them mentioned in README.md.

### Countermeasure

I installed these packages using brew.

```
brew install glew
brew install glfw3
brew install pkg-config
```

After this build worked smoothly.

I don't know what I did is correct or not. But it wll help if all the dependencies are mentioned somewhere in README.

Start in README.md and the example folder, then run make on macOS to confirm the missing dependencies and compare the build requirements with the reported GLEW, GLFW3, and pkg-config packages. Done means the README clearly lists the dependencies and setup needed to build the File Browser example.

Written by the indexing model from the issue text.