Document custom BBB to Transmitter cable creation

[ShadowLight8]

Create a custom cable to go from the BBB to the Transmitter. Accounting for the PWM line to stay away from the SDA and SCL lines as much as possible. Document the process.

[ShadowLight8]

Found out that a bad GND wire was causing all kinds of issues. Will remake cable and update README.md.

[ShadowLight8]

Hold on this for a bit to get BBB support further along