Add link to Open manual in the Menu

[polly64]

Hi @mdsimpson42 could you add link to Colouring Cities Open Manual/Wiki in the Menu https://github.com/colouring-cities/manual/wiki above View Showcase coming soon - and below it replace the showcase text with ' Case study showcase' (not yet activated'

[mdsimpson42]

Have added this to Colouring London, will copy across when I resume work on Colouring Core