A few docs installed to the wrong location

[orlitzky]

After 8e31a44d and 746f8e8 we are getting those three files installed to e.g. /usr/README.md. Since GNUInstallDirs is in use, DOCDIR is probably the best place for them, instead of DESTINATION . (Disclaimer: not a CMake expert)

If we are voting, I would also include

• AUTHORS
• CITATION.cff
• CODE_OF_CONDUCT.md
• CONTRIBUTING.md
• FEATURES.md

in the list of plain-text docs to install.

[jajhall]

What's your view of this, @galabovaa ?