Add something about calendars to the xCDAT documentation

[jypeter]

@tomvothecoder We have ran into pandas time range limitations this morning (Timestamp limitations when using a compliance checker script supplied by the ISMIP6 project

I have checked the xcdat site about this, and found that you have luckily already taken care of this problem. So has xarray

I have then checked what the xarray site was saying about the exotic calendars our community uses (by searching the site for noleap, and it turns out that they don't say much more than listing the supported calendars

Some added-value of the xCDAT documentation could be to have a page covering the different calendars our community uses, why we use them, and how to deal with them using xCDAT. I have searched noleap on the xCDAT site and you don't get much more information that it is supported by cftime.

There is not much more information about noleap on the cftime site. Basically, the only information is on the 4.4.1. Calendar section of the CF convention website. This makes sense, but I'm not sure many users will go there, when (hopefully) using xCDAT as an intelligent wrapper around all this time/calendar stuff.

That's why a page about this in the xCDAT documentation would be nice (especially if people find it in Google).

The minimum information would be to have a Where do I get information about calendars question of the FAQ page, leading to 4.4.1. Calendar

[tomvothecoder]

Nice idea @jypeter. Including your suggestions, I'll think of ways to clearly convey calendar information.

[tomvothecoder]

This idea is addressed by #383!