add build instructions

README.md

@@ -139,4 +139,11 @@ It is not really natural, but  I can't think of a better way... \
 I try to find the best compromise between the easiness of the mappings and of the actual code
 
 While c++ templates are fun, useful, and very powerful, \
-coding another program that writes repetitive code for you would give way more possibilites
+coding another program that writes repetitive code for you would give way more possibilites
+
+### Building example dll
+Use visual studio or install cmake and run :
+```
+cmake -B ./Build
+cmake --build Build --target MetaJNI --config Release
+```