component/kumofs: document that we need gcc 5.5

kumofs does not build with gcc 6

component/kumo/buildout.cfg

@@ -29,3 +29,4 @@ environment =
   LD_LIBRARY_PATH=${gcc:location}/lib:${gcc:location}/lib64
   LDFLAGS=-Wl,-rpath=${gcc:location}/lib -Wl,-rpath=${gcc:location}/lib64 -L${zlib:location}/lib -L${openssl:location}/lib -Wl,-rpath=${tokyocabinet:location}/lib -Wl,-rpath=${messagepack:location}/lib -Wl,-rpath=${zlib:location}/lib -Wl,-rpath=${openssl:location}/lib
   PATH=${gcc:location}/bin:${patch:location}/bin:%(PATH)s
+# KumoFS is known not to build with gcc>6, so we use our own gcc which is 5.5
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