document '%f' microsecond formatter (appears to come from Python, not glibc?)

[mroth]

I see a %f microsecond modifier was added in 720f6bf.

I would like to suggest possibly documenting this in the readme and Go documentation.

The readme currently mentions the glibc extensions, and both the readme and g docs link to https://linux.die.net/man/3/strftime for full formatter documentation. As far as I can tell though, it appears %f is not included in those glibc strftime extensions?

From what I can tell via web searches, it may come from the python datetime library, which in its documentation points at the C89 reference ("The following is a list of all the format codes that the 1989 C standard requires, and these work on all platforms with a standard C implementation"). However, the %f format specifier does not appear to actually appear in the C standard, even after the C99 extensions: https://en.cppreference.com/w/c/chrono/strftime. I can't find a reference to it in any other strftime implementation outside of Python, so I guess that may be where it originated?

Either way, it's useful, but I only found it via the source code, so I'd recommend documenting it for posterity. (I can make a PR if you prefer, but not sure how you would like to explain it in the docs.)

[itchyny]

Thanks for reporting. The original intention was to support microseconds in gojq, after seeing jqlang/jq#1409. Also I looked imperfectgo/go-strftime which supports this directive, but I failed investigate its origin. Though this library was intended not to include an extension like this, but %f is already used by the dependants, so hardly to drop support. I'll add a note in README.md.