Readme claims backtraces captured "if the underlying error type does not already provide its own"

This is true inasmuch as the sentence uses "if" and not "if and only if", but I was misled by the phrasing.

IIUC, anyhow can only check to see if the underlying error provides a backtrace if [the unstable `error_generic_member_access` feature is enabled](https://github.com/dtolnay/anyhow/blob/427c0bb0f34d5da378112f6a19db9efebfd0e40a/src/backtrace.rs#L38).

If so, I can do a quick PR to say this in the readme. It's only a sentence.