Added logo and edited README

[phlaster]

Hello once again! I retouched initial Ulam spiral logo:

• Now it has fewer cycles, less visual noise for an icon;
• Now spiral has 3 main colors (purple, green and red), blue migrated to letter 'i';
• Btw, now there is a package name on the left of the spiral (JuliaMono font should do it);
• White outline makes the logo compatible with dark mode;
• I put everything in documents/src/assets and named the logo appropriately, so the Documenter should render the logo automatically (or at least I hope so, as far as I read in guides).

Now it's the maintainer, who recompiles the docs, I guess..?

Have a good one!

[phlaster]

Oooff, I messed up, the PNGs somehow rendered with dark background instead of alpha channel! Actually, there are no need in them anyway SVGs do the job. Do I need to edit the request? (sry for the mess)

[oscardssmith]

No problem! You should be able to just delete the files from the pull request. (or if you get lost in git, close it and make a new one)