Update limitation on extern variables

Author: kornilova203

README.md

@@ -112,6 +112,7 @@ There are multiple unsupported cases that should be considered when generating b
 3. There is no way to reuse already generated bindings.  
     Bindgen outputs bindings also for headers that were included in a given header.
 4. Type qualifiers `const`, `volatile` and `restrict` are not supported.
+5. Extern variables are read-only. 
 
 ## License