Added line to include test requirement

[mattshu]

I'm still learning C (and PRs) so my skills might not be there yet — but I stumbled hard on the very first test (test_reading_empty_buffer_fails()) because I didn't realize I had to set errno to a special value.

I thought this change could serve as a small hint for newbies like me to consider how errors are raised in C. I might be overthinking it, though!

[ryanplusplus]

Hey @mattshu, these docs are generated from the upstream/common problem specifications. To add track specific data we need to either add an append. Here's an example of an append on this track:

c/exercises/practice/gigasecond/.docs/instructions.append.md

Line 4 in c8bad63

The [`time.h`][time.h] library gives you two functions to do this.

[mattshu]

Thank you for the info @ryanplusplus!