Add warning when using symfony/symfony directly

Author: nicolas-grekas

symfony/symfony/3.3/manifest.json

@@ -0,0 +1,2 @@
+{
+}

symfony/symfony/3.3/post-install.txt

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+<bg=yellow;fg=black>                                                                                 </>
+<bg=yellow;fg=black> Using "symfony/symfony" in your dependencies is incompatible with Symfony Flex! </>
+<bg=yellow;fg=black>                                                                                 </>
+
+  * <fg=blue>Instead</> use individual <comment>symfony/*</comment> packages.