Fixed not found images on README.md

[sav-valerio]

I completed the 3 steps below:

• I've read the Contribution Guide.
• I've updated the documentation. (refer to this for how to do so). (not needed)
• I enjoyed my time contributing and making developer's life easier :)

I added the images to the repository itself, so we don't need to deal with external providers.
Of course you first need to merge the Pull Request to the master branch, or the images won't appear.

[sav-valerio]

This PR just disappeared in the dark.
Main logo has been fixed, however the "docs" logo is still missing.