SF.11, Header files should be self-contained (#1181)

* SF.11 Header files should be self-sufficient added

* PR feedback addressed

* explain more

* update reason baed on PR feedback

* add VS code dir to .gitignore

* more PR feedback

.gitignore

@@ -4,3 +4,6 @@ node_modules
 _site
 scripts/python/__pycache__
 scripts/python/*.pyc
+
+# VS Code
+.vs/

CppCoreGuidelines.md

@@ -18207,6 +18207,7 @@ Source file rule summary:
 * [SF.8: Use `#include` guards for all `.h` files](#Rs-guards)
 * [SF.9: Avoid cyclic dependencies among source files](#Rs-cycles)
 * [SF.10: Avoid dependencies on implicitly `#include`d names](#Rs-implicit)
+* [SF.11: Header files should be self-contained](#Rs-contained)
 
 * [SF.20: Use `namespace`s to express logical structure](#Rs-namespace)
 * [SF.21: Don't use an unnamed (anonymous) namespace in a header](#Rs-unnamed)
@@ -18614,6 +18615,27 @@ This rule against implicit inclusion is not meant to prevent such deliberate agg
 Enforcement would require some knowledge about what in a header is meant to be "exported" to users and what is there to enable implementation.
 No really good solution is possible until we have modules.
 
+### <a name="Rs-contained"></a>SF.11: Header files should be self-contained
+
+##### Reason
+
+Usability, headers should be simple to use and work when included on their own.
+Headers should encapsulate the functionality they provide.
+Avoid clients of a header having to manage that header's dependencies.
+
+##### Example
+
+    #include "helpers.h"
+    // helpers.h depends on std::string and includes <string>
+
+##### Note
+
+Failing to follow this results in difficult to diagnose errors for clients of a header.
+
+##### Enforcement
+
+A test should verify that the header file itself compiles or that a cpp file which only includes the header file compiles.
+
 ### <a name="Rs-namespace"></a>SF.20: Use `namespace`s to express logical structure
 
 ##### Reason