Consider adding dev editor config, setup guide, styler, styler hooks

[brookslogan]

Consider

• adding some editor configuration files to help maintain a consistent style
• adding a development guide for the same purpose + discuss branch&PR setup, running check(), etc.
• running through styler and adding styler hooks

For editor config, maybe the following for Emacs ESS:

.dir-locals.el

((ess-r-mode
  . ((ess-style . RStudio) ; seems to match or is close to existing style
     )))

and guidance to consider adding
.dir-locals-2.el

((ess-r-mode
  . ((comment-add . 0) ; to get commenting commands to avoid "##" in order to
                       ; match this style until
                       ; https://github.com/emacs-ess/ESS/issues/996 is
                       ; resolved, but triggering a may-not-be-safe message
     )))

and to instruct Emacs that this is safe when it asks (--- have this in guidance to avoid scary "safety" messages for regular users browsing code).

[dshemetov]

Styler was added in #425. Development guide was added a while ago. Adding editor config in #538 .