add a link to the development website

[avallecam]

The epidemics package has two websites:

• https://epiverse-trace.github.io/epidemics/
• https://epiverse-trace.github.io/epidemics/dev/

We can add this paragraph to README, inspired by https://epiforecasts.io/EpiNow2/#resources

The package has two websites: one for the stable release version on CRAN, and another for the version in development. The development website may contain experimental features and information not yet available in the stable release.

cc: @adamkucharski