Added documentation to warn for #233

Since it was not as apparent as I thought it is I added some
documentation warning to set the same flags each time you
include "nuklear.h" except `NK_IMPLEMENTATION` which is only
allowed to be defined once in *one* source file.

Readme.md

@@ -36,6 +36,8 @@ The implementation mode requires to define  the preprocessor macro
 #define NK_IMPLEMENTATION
 #include "nuklear.h"
 ```
+IMPORTANT: Every time you include "nuklear.h" you have to define the same optional flags.
+This is very important not doing it either leads to compiler errors or even worse stack corruptions.
 
 ## Gallery
 ![screenshot](https://cloud.githubusercontent.com/assets/8057201/11761525/ae06f0ca-a0c6-11e5-819d-5610b25f6ef4.gif)

nuklear.h

@@ -42,6 +42,9 @@ USAGE:
     Also optionally define the symbols listed in the section "OPTIONAL DEFINES"
     below in header and implementation mode if you want to use additional functionality
     or need more control over the library.
+    IMPORTANT:  Every time you include "nuklear.h" you have to define the same flags.
+                This is very important not doing it either leads to compiler errors
+                or even worse stack corruptions.
 
 FEATURES:
     - Absolutely no platform dependend code