Add po/R-potools.pot to this package

[HenrikBengtsson]

For someone trying to wrap their head around what files are being produced and where, I think it could helpful to include:

po/R-potools.pot

and an example of a language translation, e.g.

po/R-es.pot

to the potools package itself.

[HenrikBengtsson]

This could also be use to examplify how to use LANGUAGE, LANG, and LC_MESSAGES, and when those needs to/can be set, e.g. before or after R starts.

It can also be use to illustrate what happens if one of the messages is not translated.

These are things that end users as well as developers are gonna wonder about.

[hadley]

If you use devtools, you might want the dev version of pkgload which will automatically reload the .mo files: r-lib/pkgload#171

To confirm that the translations are working, you might be interested in with_language(), r-lib/withr#180, or the equivalent for base R, https://bugs.r-project.org/show_bug.cgi?id=18055.