add more detail about namespaces in extension-methods (#1667)
add more detail about namespaces in extension-methods (#1667)

* add more detail about namespaces

* Update concepts/extension-methods/about.md

Co-authored-by: Erik Schierboom <6e321e24be838c7616721fdb824f9c65569cbfd8@hotmail.com>

Co-authored-by: Erik Schierboom <6e321e24be838c7616721fdb824f9c65569cbfd8@hotmail.com>
exercism/xcsharp,exercism/xcsharp