Added rspamd_logo_white_bg.png to update on readme

[pakhare]

Currently on rspamd/rspamd the readme file has the logo that is not clear when dark mode is turned on. Adding this image and updating the url on rspamd/rspamd repository will solve this issue. There is another way as well, to switch between logos according to light or dark mode, but then we can not attach a link to it, or change its size with HTML. So this is the current best workaround,

[pakhare]

I'll update on the main repo as soon as this PR gets merged.

[moisseev]

@pakhare , thanks for addressing the dark mode logo issue!

IMO, instead of linking the image from another repository, it might be better to store the image directly in the rspamd/rspamd repository. That way, when the repository is cloned or viewed offline, everything stays intact. Keeping the asset in the repository ensures it remains self-contained and versioned with the rest of the project, reducing the risk of broken links if the external repository structure changes.

@vstakhov , what do you think?

[pakhare]

I had the same thought to keep it within the repository, but currently it is being loaded from https://rspamd.com/img/rspamd_logo_black.png moreover, the repo does not contain any image assets as they are not necessary for the development, and may be that is why this decision was taken.

That said I would also suggest to keep it within the repository to avoid unnecessary resource usage at rspamd.com