Document removing the embedded backtracer

[theRookieCoder]

As suggested in google/bloaty#110 (comment) and discussed in a Reddit post, disabling the backtrace feature in std can reduce the binary size by a fixed amount of ~300 kB. This may be significant for programs aiming to be absolutely minimal, e.g. a "Hello, world!" program can be reduced in size by 83%.

This information could be added to the Abort on Panic section, as the embedded backtracer should be unused when aborting on panic!()s.

[T-256]

You are actually removing backtrace feature when using build-std feature. So perhaps it's worth add note on that section what's default value build-std-features and what happens when override that value.