Add full URLs to the website location of Information files

[maurolepore]

In R-devel, CRAN now checks for full URLs when doing a new submisison (see also r-lib/usethis#772).

TODO

Change relative to full links at https://github.com/forestgeo/fgeo.plot/blob/master/README.Rmd

Bad:

* [Getting help](SUPPORT.md).
* [Contributing](CONTRIBUTING.md).
* [Contributor Code of Conduct](CODE_OF_CONDUCT.md).

Good:

* [Getting help](https://forestgeo.github.io/fgeo.plot/SUPPORT.html).
* [Contributing](https://forestgeo.github.io/fgeo.plot/CONTRIBUTING.html).
* [Contributor Code of Conduct](https://forestgeo.github.io/fgeo.plot/CODE_OF_CONDUCT.html).

Rejection email

Sent: Wednesday, May 29, 2019 3:40 AM
Subject: Re: CRAN submission fgeo.plot 1.1.7

Thanks, we see:

   Found the following (possibly) invalid file URIs:
     URI: .github/SUPPORT.md
       From: README.md
     URI: .github/CONTRIBUTING.md
       From: README.md
     URI: .github/CODE_OF_CONDUCT.md
       From: README.md

These files do not exist.
Better add full URLs to the github location of these files?

Please fix and resubmit.

Best,
Uwe Ligges