Format + complete PCB README

[cmdoret]

I just linked individual README files into the github-pages website. We should ensure each README is complete and has a consistent formatting

• Each README should be located in beemos/board-name/README.md

• [x] counter-i2c

• [x] master-simple

• [ ] scale-i2c

• [x] weather-i2c

• They should have well defined sections (e.g. Description, Components, Performances)

• [x] counter-i2c

• [ ] master-simple

• [ ] scale-i2c

• [x] weather-i2c

• They should include a link to the main github-pages website at the top

• [x] counter-i2c

• [x] master-simple

• [ ] scale-i2c

• [x] weather-i2c