Updated test case documentation

docs/test_cases/t20006.md

@@ -0,0 +1,54 @@
+# t20006 - Class template specialization basic sequence diagram
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t20006_sequence:
+    type: sequence
+    glob:
+      - ../../tests/t20006/t20006.cc
+    include:
+      namespaces:
+        - clanguml::t20006
+    using_namespace:
+      - clanguml::t20006
+    start_from:
+      - function: "clanguml::t20006::tmain()"
+```
+## Source code
+File t20006.cc
+```cpp
+#include <string>
+
+namespace clanguml {
+namespace t20006 {
+
+template <typename T> struct A {
+    T a_int(T arg) { return arg + 1; }
+    T a_string(T arg) { return arg + "_string"; }
+};
+
+template <typename T> struct B {
+    T b(T arg) { return a_.a_int(arg); }
+    A<T> a_;
+};
+
+template <> struct B<std::string> {
+    std::string b(std::string arg) { return a_.a_string(arg); }
+    A<std::string> a_;
+};
+
+void tmain()
+{
+    B<int> bint;
+    B<std::string> bstring;
+
+    bint.b(1);
+    bstring.b("bstring");
+}
+}
+}
+```
+## Generated UML diagrams
+![t20006_sequence](./t20006_sequence.svg "Class template specialization basic sequence diagram")