Update Readme to reflect all changes required to integrate with an existing symfony project

[StephenGillCoop]

• [ ] Pending Invitations for the Tenant: /vendor/tahoelimited/multi-tenancy-bundle/Resources/views/Invitation/index.html.twig
• [ ] Switch Tenant to another tenant this logged in user has access to: /vendor/tahoelimited/multi-tenancy-bundle/Resources/views/Organization/switch_dropdown.html.twig
• [ ] Pending Invitations for a specific user: /vendor/tahoelimited/multi-tenancy-bundle/Resources/views/OrganizationUser/_invitations.html.twig
• [ ] ???: /vendor/tahoelimited/multi-tenancy-bundle/Resources/views/OrganizationUser/_organization_users.html.twig
• [ ] List of users with access to this tenant: /vendor/tahoelimited/multi-tenancy-bundle/Resources/views/OrganizationUser/index.html.twig

Also explain how the concepts work e.g. how invitations work (on the wiki page or a longer readme?)