Add section in README about custom configs such as build properties

[simojo]

Currently, if someone needs to manually tweak compiler flags or other platform-specific properties, there is no documented way to do so. It would be helpful to add something so that people in need of this could reference it. The owner of this repo has proposed to override the fixupPhase of a given arduino platform's derivation instead of creating an official method to do so as I proposed in #6.

Hi @simojo, thanks for taking the time to create this PR.
    
I don't think this is needed however, if you want to modify a platform you can do something like:
    
    
  ```
  `platforms.rp2040.rp2040."version".overrideAttrs (self: super: {
    fixupPhase = "...";
  })
  ```
  
  Which is what we do for something similar. I would accept a PR that updates the README to include this information 😄
  
  _Originally posted by @bouk in https://github.com/bouk/arduino-nix/issues/6#issuecomment-2512025493_

[srd424]

esp8266 needs something similar:

(platforms.esp8266.esp8266."3.1.2".overrideAttrs {
  postInstall = "touch $out/$dirName/cores/esp8266/CommonHFile.h";
})

(it also needs python3 and possibly pyserial in the path, but I've not got around to looking at that yet.)

I know @bouk wants to keep this repo relatively uncluttered - would it maybe make sense to either make arduino-indexes contain actual nix overlays rather than json so specific packages could be overriden? Alternatively that could be done in one or more new repos..