docs: Updating README with repository details. (#101)

* docs: fleshing out the readme

Making it consistent with frontend-component-footer and much more detailed.

* fix: Removing unused variable, siteName

Also fixing logout URL in development.  (We fixed this in a number of other repositories already, but somehow this one snuck through)

src/Header.jsx

@@ -79,7 +79,6 @@ function Header({ intl }) {
   const props = {
     logo: config.LOGO_URL,
     logoAltText: config.SITE_NAME,
-    siteName: config.SITE_NAME,
     logoDestination: `${config.LMS_BASE_URL}/dashboard`,
     loggedIn: authenticatedUser !== null,
     username: authenticatedUser !== null ? authenticatedUser.username : null,