README: added note about GCCv6 problem

README.md

@@ -63,6 +63,13 @@ RIOT is based on a microkernel architecture, and provides features including, bu
 * You want to start the RIOT? Just follow our [Getting started documentation](https://github.com/RIOT-OS/RIOT/wiki/Introduction)
 * The RIOT API itself can be built from the code using doxygen. The latest version is uploaded daily to http://riot-os.org/api.
 
+## KNOWN ISSUES
+* With latest GCC version (>= 6) platforms based on some ARM platforms will
+  raise some warnings, leading to a failing build
+  (see https://github.com/RIOT-OS/RIOT/issues/5519).
+  As a workaround, you can compile with warnings not being treated as errors:
+  `WERROR=0 make`
+
 ### USING THE NATIVE PORT WITH NETWORKING
 If you compile RIOT for the native cpu and include the `netdev2_tap` module, you can specify a network interface like this: `PORT=tap0 make term`