Update readme.md (mostly firmware related changes)

[AntonEvmenenko]

List of changes:

1. Slightly adjusted the formatting; images are now properly aligned
2. Changed the order of hardware revisions. Previously it was out of order: V1.5 → V2.0 → V1.0. Now it's V2.0 → V1.5 → V1.0. The most recent revision is at the top of the list; seems logical and correct to me
3. Removed the mention of firmware from the "Repository Structure" section, since all firmware is located in other repos
4. Added a separate "Firmware" section and placed it above the hardware revisions. Just because it's a new and very small section; if it's at the bottom, nobody will notice it