Minimal {styler} version requirement

[lorenzwalthert]

{styler} won't ever bring up interactive prompt to create a cache (as requested by {R.cache}) as of version 1.5.1. For this reason, the corresponding README explanation becomes obsolete if you add a minimal version requirement on {styler} (1.5.1), which I suggest to do.
Also see r-lib/styler#819.

[renkun-ken]

Looks like the new R.cache will by default uses a global cache root path for the user (~/.cache/R/R.cache). Then there seems to be no need to mention the cache issue in README at all.

[lorenzwalthert]

The difference is that {R.cache} uses a standard location that does not require user permission according to CRAN policies.