Add POD and KLM guides

[gerritholl]

The POD and KLM guides are increasingly hard to find online. Add own
copy so they're never lost.

[mraspaud]

That's too much data to have in the repo. Can we put it in the wiki instead ? https://github.com/pytroll/pygac/wiki

[gerritholl]

Isn't the wiki also a repo? Ergo does that make a difference?

[mraspaud]

Yes, it's a repo, but not this repo. So we won't clutter the code repo with large files (which will affect performance of git operations). The wiki on the other hand doesn't have much git activity.

[gerritholl]

Not here.