Add icon policy documentation

[Bosch-0]

I created some documentation as to how icons should be prepared, optimized, styled, contributed and attributed within Bitcoin Core. Having guidelines for iconography helps with making applications more consistent and efficient.

The current icon set does not yet follow this policy but I will be updating them in future PRs. To see the current icon set using this policy see this figma source file.

[jonasschnelli]

Concept ACK. Great work.
My only question would be, it this should belong to the root docs/ directory or if we should place it in src/qt/res/.

[Bosch-0]

Thanks for the suggestions :)

[RandyMcMillan]

I like the idea of putting it in the /res folder.
Maybe even just name it README.md if it lands in the /res folder?
This may be a good place to add logo/branding specification notes in the future.

[RandyMcMillan]

This may be helpful in getting this PR to pass.
The parent commits are probably too stale.

#139 (comment)

[Bosch-0]

My only question would be, it this should belong to the root docs/ directory or if we should place it in src/qt/res/.

Yes this is probably a better location for this, I'll move the file and squash the commits

[jarolrod]

You should probably squash the commits

[RandyMcMillan]

I still think it should be named README.md
There is other documentation we could add to this later. Why not be proactive?

Otherwise ACK

[Bosch-0]

Moved #178