Define Serial API

[MAhmedDev]

https://github.com/Niuslar/Multichannel-Temperature-Control/wiki/Serial-API

[Vavat]

@FrozenOxide, is what I've written in the wiki enough to close this ticket or did you have more in mind?

[MAhmedDev]

I think we should state all the JSON commands explicitly e.g

{
"target_temperature":37.2
}