README: add note about roadmap and google group
README: add note about roadmap and google group

Closes #297

zlabjp/dex,andrewstuart/dex,zlabjp/dex,Tecsisa/dex,andrewstuart/dex,Tecsisa/dex,zlabjp/dex,Tecsisa/dex,andrewstuart/dex