docs(INSTALL): update install docs with workaround for fedora 25

INSTALL.md

@@ -556,13 +556,19 @@ echo '/usr/local/lib/' | sudo tee -a /etc/ld.so.conf.d/locallib.conf
 sudo ldconfig
 ```
 
-
 ### Compile qTox
 
 **Make sure that all the dependencies are installed.**  If you experience
 problems with compiling, it's most likely due to missing dependencies, so please
 make sure that you did install *all of them*.
 
+If you are compiling on Fedora 25, you must add libtoxcore to the
+`PKG_CONFIG_PATH` environment variable manually:
+
+```
+export PKG_CONFIG_PATH="$PKG_CONFIG_PATH:/usr/local/lib/pkgconfig"
+```
+
 Run in qTox directory to compile:
 
 ```bash