Add icons for more documentation files

[Semphriss]

I've noticed that although many common documentation files have a special icon, many others, like SECURITY or CODE_OF_CONDUCT, don't. I propose to add some of them.

Note: As I'm unfamiliar with the selection criteria, I've added rather more icons than less in case they might be interesting; I'm open to leave some of them out if they are considered irrelevant. I saw in #642 that counting files across repositories on GitHub was one criterion; I provided my own findings about those files for convenience:

Filename	(none)	.md	.txt	Proposed icon
CODE_OF_CONDUCT	719k	1.2M	2k	(New icon)
CODINGSTYLE	72k	3k	839	(New icon)
CODING_STYLE	141k	16k	4k	(New icon)
NEWS	6M	165k	454k	Same as CHANGELOG
ROADMAP	183k	86k	6k	Same as TODO
INSTALL	22M	330k	426k	Same as COMPILING
SECURITY	16M	1M	111k	(New icon)
AUTHOR	2.4M	27k	26k	(New icon)
AUTHORS	5M	659k	666k	(New icon)
CREDIT	2.9M	21k	70k	(New icon)
CREDITS	1.4M	38k	153k	(New icon)

The proposed icons:

• Code of conduct:
• Coding style:
• Security:
• Authors / credit:

The current icon selection is only my proposition (and the expression of my poor talents); I am open to change the icons/pairings if requested.

[Semphriss]

Is there any interest in this PR? I can fix the conflicts, but if it won't be merged anyways, I can close it instead.