Potential documentation updates

[filmor]

There have been some changes in rustler that are not accounted for in the docs, yet:

• The derive feature flag is a no-op since 0.34 (and I'd like to remove it in the future)
• The macOS linkage override is not necessary anymore since 0.34 (I think)
• The default feature set is now simply nif_version_2_15 and will likely stay at that for a while, and I'd appreciate it if the documentation of RustlerPrecompiled could not advise to disable it if it's not necessary. Maybe you could keep a table of "rustler version <=> minimal nif version" around?

[philss]

I should work on that soon. Thanks for pointing out the issues!