Warn about `RDML::rdmlEdit()` dependence on SUGGESTS packages, e.g. shinythemes, etc.

[ramiromagno]

To avoid this unexpected warning and error, the dependence on shinythemes could be clarified in the README.

> RDML::rdmlEdit()
Loading required package: shiny

Listening on http://127.0.0.1:7933
Warning: Error in library: there is no package called ‘shinythemes’
  74: <Anonymous>