Add more complete documentation

[thomas-touhey]

Hopefully that's good enough. :)

Closes: #18

[jepler]

Thank you! I really appreciate this, it's a non-trivial amount of effort you've made. I do have some feedback which I hope is not too nit-picky.

In several places you use the term "semester", for instance stating that a leap second can occur "at the end of the semester". To me this is unusual usage (as a speaker of American English, "semester" is usually used to refer to school terms, in which case it rarely corresponds to 6 months. For instance when I was a kid, the "fall semester" of the city schools would run from when school started in early September until early January for some reason).

If it is usual usage for talking about leap seconds in English I'm happy to leave it, but for instance the word "semester" doesn't appear in the english language wikipedia page https://en.wikipedia.org/wiki/Leap_second —so I'm guessing it's something that feels natural in French but doesn't quite come over right to (American) English. If you don't mind, can you just say "every six months" for the frequency of the Bulletin?

As for the timing of leap seconds, now that there is documentation, I think it should stress that the leap second occurs at the end of the minute 23:59 UTC; the local time during a leap second could be (for example in parts of the US) 16:59:60 or even (if my math is right; UTC+0845 is a time zone used for part of the year in part of Australia) 08:44:60 the next day (e.g., January 1).

Thanks again, I'm excited about this addition to the docs.

[thomas-touhey]

Thanks for the feedback, edits are made :)

[jepler]

Thank you!