Document clear distinction between immediate & fallback aborts

Could be important in some safety scenarios, and explains why fallback implementation prints to stderr.

The README.md file could use some cleanup after this.

Forgot to number a version v0.1.5 and made a silly mistake in v0.1.6 which I fixed in the next commit. I yanked v0.1.6 from crates.io because it has no changes except a missing feature-declaration.
I also combined the release notes for v0.1.6 and v0.1.7.