Add documentation for "git commit --date=now"

[johnlinp]

There are some special strings that we can pass to --date in git commit, e.g. now, yesterday, noon, tea...

Personally I think --date=now is the most useful because I will use it with git commit --amend. I guess we don't have to make an exhaustive list of special strings in the documentation, but I think we can at least mention now.

[jpninanjohn]

I can do this. Can someone please point me to where the docs are kept?

[dscho]

Can someone please point me to where the docs are kept?

Gladly: https://github.com/git/git/tree/v2.23.0/Documentation/

You will probably see very quickly where the patch needs to go ;-)

[jpninanjohn]

#408

I have raised a pull request for it. I am not sure if I have fulfilled the commit message requirements correctly. Do tell if there is anything I have to correct.