Fix mistakes in README, improve formatting

[mpaw]

• Fix mistake where the instructions state that the red connections must be connected when it actually should say blue connections must be connected
• Improve instructions for manual calibration and add a table with commands and how much they offset the values
• Fix grammatical errors and improve overall instructions

[mpaw]

@remrc Let me know if you want me to make any changes

[mpaw]

Here's a preview of the README with my changes
https://github.com/remrc/Self-Balancing-Cube/blob/4bd74179ce98a71107d4cb61bc073f48a263a718/README.md

[remrc]

Where I say that the blue connections are not required?

[mpaw]

@remrc Sorry, I quoted you incorrectly. What you actually said is

The red connections you see in the schematic must be connected!

But it should be

The blue connections you see in the schematic must be connected!

[mpaw]

I updated the PR description to clarify what I meant