Update README.md

[davidxll]

There are repeated and missing steps in the README.md

1. Sections of line 3 and 25 are the same.
2. Two download sections (lines 61 and 71) with partially same information.
3. Miss to mention to open the tutorial file.
4. Pictures doesn't show which one is the "mecanum wheels" and the "ordinary wheels".

Thank you

PS: It may be good to have helpful links, for example: How to flash/setup a Raspberry Pi, or a link to the official website.