Bad link to "entr" utility

[rwv37]

The "Using shpec" section of README.md has the following sentence: "If you'd like your tests to run automatically when they change, we recommend the entr utility". The "entr" is a link. That link is now a redirect to some SEO site.

[hlangeveld]

Either we should remove the section, or link to https://github.com/eradman/entr/ instead.