Add cfamily to rule S6418

rules/S6418/cfamily/metadata.json

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+{
+}

rules/S6418/cfamily/rule.adoc

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+FIXME: add a description
+
+// If you want to factorize the description uncomment the following line and create the file.
+//include::../description.adoc[]
+
+== Why is this an issue?
+
+FIXME: remove the unused optional headers (that are commented out)
+
+//=== What is the potential impact?
+
+== How to fix it
+//== How to fix it in FRAMEWORK NAME
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,cpp,diff-id=1,diff-type=noncompliant]
+----
+FIXME
+----
+
+==== Compliant solution
+
+[source,cpp,diff-id=1,diff-type=compliant]
+----
+FIXME
+----
+
+//=== How does this work?
+
+//=== Pitfalls
+
+//=== Going the extra mile
+
+
+//== Resources
+//=== Documentation
+//=== Articles & blog posts
+//=== Conference presentations
+//=== Standards
+//=== External coding guidelines
+//=== Benchmarks