Updated test case documentation

docs/test_cases/t20018.md

@@ -0,0 +1,44 @@
+# t20018 - Recursive template sequence diagram test case
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t20018_sequence:
+    type: sequence
+    glob:
+      - ../../tests/t20018/t20018.cc
+    include:
+      namespaces:
+        - clanguml::t20018
+    using_namespace:
+      - clanguml::t20018
+    start_from:
+      - function: "clanguml::t20018::tmain()"
+```
+## Source code
+File t20018.cc
+```cpp
+#include <iostream>
+
+namespace clanguml {
+namespace t20018 {
+
+template <int N> struct Factorial {
+    static const int value = N * Factorial<N - 1>::value;
+
+    static void print() { Factorial<N - 1>::print(); }
+};
+
+template <> struct Factorial<0> {
+    static const int value = 1;
+
+    static void print() { std::cout << "Hello world\n"; }
+};
+
+void tmain() { Factorial<5>::print(); }
+}
+}
+```
+## Generated UML diagrams
+![t20018_sequence](./t20018_sequence.svg "Recursive template sequence diagram test case")