Update README with RAM suggestions for ESPHome

[deviantintegral]

I discovered that compiling ratgdo firmware requires at least 2GB of memory. The thing is, it's very jumpy, and if you watch memory use it will go between ~200MB and 1.5GB+ quickly as cc processes spawn and finish. In my case, I was lucky because the memory use was always significantly more than Home Assistant, so the OOM killer would kill the compiler.

Let's warn users about this so they don't accidentally take their home offline.